diff --git a/Cargo.lock b/Cargo.lock
index 58971a6a3..39c5cdc33 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -71,7 +71,7 @@ checksum = "366ffbaa4442f4684d91e2cd7c5ea7c4ed8add41959a31447066e279e432b618"
 [[package]]
 name = "acadrust"
 version = "0.4.0"
-source = "git+https://github.com/HakanSeven12/acadrust?branch=main#ad507470661689a85a999166daf7f22ee507e63c"
+source = "git+https://github.com/HakanSeven12/acadrust?branch=main#3dbae0556208221822257ead37a8d445dcc7d768"
 dependencies = [
  "ahash 0.8.12",
  "anyhow",
@@ -3410,7 +3410,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "0b577e2d69827c4740cba2b52efaad1c4cc7c73042860b199710b3575c68438d"
 dependencies = [
  "bytecount",
- "memchr 1.0.2",
+ "memchr 2.8.2",
  "nom 8.0.0",
 ]
 
diff --git a/src/app/update/dialog.rs b/src/app/update/dialog.rs
index 84c967220..89df077c8 100644
--- a/src/app/update/dialog.rs
+++ b/src/app/update/dialog.rs
@@ -23,6 +23,20 @@ use iced::{mouse, Point, Task};
 
 impl OpenCADStudio {
     pub(in crate::app) fn open_save_dialog_window(&mut self, tab_idx: usize) -> Task<Message> {
+        // Default the format dropdown to the loaded file's version so a
+        // round-trip preserves it (e.g. an R2004 drawing offers "DWG 2004",
+        // not "DWG 2018"). The save still falls back to the source version if
+        // the user picks an incompatible one (dwg_save_version).
+        let is_dxf = self.tabs[tab_idx]
+            .current_path
+            .as_ref()
+            .and_then(|p| p.extension())
+            .and_then(|e| e.to_str())
+            .map(|e| e.eq_ignore_ascii_case("dxf"))
+            .unwrap_or(false);
+        let doc_version = self.tabs[tab_idx].scene.document.version;
+        self.save_dialog_format = crate::io::format_for_version(doc_version, is_dxf);
+
         // Pre-fill filename and folder from current path or defaults.
         if let Some(p) = &self.tabs[tab_idx].current_path.clone() {
             if let Some(name) = p.file_name() {
@@ -244,9 +258,11 @@ pub(super) fn on_ribbon_tool_click(&mut self, tool_id: String, event: ModuleEven
     }
 
     pub(super) fn on_unsaved_picked_save_path_some(&mut self, path: std::path::PathBuf) -> Task<Message> {
-                let (_, version) = crate::io::parse_save_format(&self.save_dialog_format);
+                let (ext, version) = crate::io::parse_save_format(&self.save_dialog_format);
                 match self.pending_close.take() {
                     Some(crate::app::PendingClose::Tab(idx)) => {
+                        // Fall back to source version for version-locked data.
+                        let version = self.dwg_save_version(idx, ext, version);
                         match crate::io::save_as_version(
                             &self.tabs[idx].scene.document,
                             &path,
@@ -268,6 +284,8 @@ pub(super) fn on_ribbon_tool_click(&mut self, tool_id: String, event: ModuleEven
                     }
                     Some(crate::app::PendingClose::Quit) => {
                         let i = self.active_tab;
+                        // Fall back to source version for version-locked data.
+                        let version = self.dwg_save_version(i, ext, version);
                         match crate::io::save_as_version(
                             &self.tabs[i].scene.document,
                             &path,
diff --git a/src/app/update/file.rs b/src/app/update/file.rs
index a6d42d556..84ff81689 100644
--- a/src/app/update/file.rs
+++ b/src/app/update/file.rs
@@ -518,6 +518,43 @@ pub(super) fn on_open_file(&mut self) -> Task<Message> {
                 Task::none()
     }
 
+    /// Resolve the effective DWG save version for `tab`.
+    ///
+    /// A drawing that carries data locked to its source DWG version (e.g.
+    /// AEC/Civil3D objects with no published spec, raw MLEADER/Surface records,
+    /// EED) can only be written losslessly in that version. When the requested
+    /// version is in a different encoding family, fall back to the source
+    /// version so nothing is dropped and the file opens cleanly elsewhere, and
+    /// note it on the command line. Non-DWG / convertible saves are unchanged.
+    /// Shared by every save path (Save, Save As, save-before-close).
+    pub(crate) fn dwg_save_version(
+        &mut self,
+        tab: usize,
+        ext: &str,
+        requested: acadrust::DxfVersion,
+    ) -> acadrust::DxfVersion {
+        if ext != "dwg" {
+            return requested;
+        }
+        let fallback = {
+            let doc = &self.tabs[tab].scene.document;
+            match doc.dwg_source_version {
+                Some(src) if src != requested && doc.has_version_locked_data(requested) => Some(src),
+                _ => None,
+            }
+        };
+        match fallback {
+            Some(src) => {
+                self.command_line.push_output(&format!(
+                    "Saved in source DWG version ({src:?}) to preserve objects that cannot be \
+                     converted to the selected version."
+                ));
+                src
+            }
+            None => requested,
+        }
+    }
+
     pub(super) fn on_save_file(&mut self) -> Task<Message> {
                 if self.read_only {
                     self.command_line
@@ -535,7 +572,17 @@ pub(super) fn on_open_file(&mut self) -> Task<Message> {
                 if let Some(path) = self.tabs[i].current_path.clone() {
                     self.tabs[i].scene.document.header.user_real1 =
                         self.tabs[i].scene.annotation_scale as f64;
-                    match crate::io::save(&self.tabs[i].scene.document, &path) {
+                    // Preserve the document's version, but fall back to the
+                    // source version if it carries version-locked data (so a
+                    // direct Save can't silently drop AEC/Civil3D objects).
+                    let ext = path
+                        .extension()
+                        .and_then(|e| e.to_str())
+                        .unwrap_or("")
+                        .to_ascii_lowercase();
+                    let requested = self.tabs[i].scene.document.version;
+                    let version = self.dwg_save_version(i, &ext, requested);
+                    match crate::io::save_as_version(&self.tabs[i].scene.document, &path, version) {
                         Ok(()) => {
                             self.command_line
                                 .push_output(&format!("Saved: {}", path.display()));
@@ -567,6 +614,10 @@ pub(super) fn on_open_file(&mut self) -> Task<Message> {
                 let i = self.active_tab;
                 sync_annotation_scale_header(&mut self.tabs[i].scene);
 
+                // Fall back to the source version for files with version-locked
+                // data (e.g. AEC/Civil3D objects) so nothing is dropped.
+                let version = self.dwg_save_version(i, ext, version);
+
                 // Native: write to the chosen path. Web: download the bytes
                 // under the chosen name (no filesystem).
                 #[cfg(not(target_arch = "wasm32"))]
diff --git a/src/io/mod.rs b/src/io/mod.rs
index f4c525535..3aa646c8a 100644
--- a/src/io/mod.rs
+++ b/src/io/mod.rs
@@ -423,6 +423,25 @@ pub fn parse_save_format(format: &str) -> (&'static str, acadrust::DxfVersion) {
     (ext, version)
 }
 
+/// Reverse of [`parse_save_format`]: the Save-dialog format string for a
+/// version + DXF/DWG choice (e.g. `AC1018, is_dxf=false` -> `"DWG 2004"`).
+/// Used to default the Save-As dropdown to the loaded file's version so a
+/// round-trip preserves it instead of silently offering "DWG 2018".
+pub fn format_for_version(version: acadrust::DxfVersion, is_dxf: bool) -> String {
+    use acadrust::DxfVersion::*;
+    let year = match version {
+        AC1032 => "2018",
+        AC1027 => "2013",
+        AC1024 => "2010",
+        AC1021 => "2007",
+        AC1018 => "2004",
+        AC1015 => "2000",
+        AC1014 => "R14",
+        _ => "2018",
+    };
+    format!("{} {}", if is_dxf { "DXF" } else { "DWG" }, year)
+}
+
 // ── Plot Style Table ──────────────────────────────────────────────────────
 
 /// Show a file-open dialog and load the selected CTB or STB file.