diff --git a/cli/src/cli_util.rs b/cli/src/cli_util.rs
index a3e7c3b76..5d278c14b 100644
--- a/cli/src/cli_util.rs
+++ b/cli/src/cli_util.rs
@@ -1914,8 +1914,8 @@ pub fn update_working_copy(
     old_commit: Option<&Commit>,
     new_commit: &Commit,
 ) -> Result<Option<CheckoutStats>, CommandError> {
-    let old_tree_id = old_commit.map(|commit| commit.tree_id().clone());
-    let stats = if Some(new_commit.tree_id()) != old_tree_id.as_ref() {
+    let old_tree_id = old_commit.map(|commit| commit.merged_tree_id().clone());
+    let stats = if Some(new_commit.merged_tree_id()) != old_tree_id.as_ref() {
         // TODO: CheckoutError::ConcurrentCheckout should probably just result in a
         // warning for most commands (but be an error for the checkout command)
         let new_tree = new_commit.merged_tree()?;
diff --git a/lib/src/working_copy.rs b/lib/src/working_copy.rs
index 34d144273..4e7753a5f 100644
--- a/lib/src/working_copy.rs
+++ b/lib/src/working_copy.rs
@@ -1524,7 +1524,7 @@ impl WorkingCopy {
     pub fn check_out(
         &mut self,
         operation_id: OperationId,
-        old_tree_id: Option<&TreeId>,
+        old_tree_id: Option<&MergedTreeId>,
         new_tree: &MergedTree,
     ) -> Result<CheckoutStats, CheckoutError> {
         let mut locked_wc = self.start_mutation()?;
@@ -1533,7 +1533,7 @@ impl WorkingCopy {
         // regardless, but it's probably not what  the caller wanted, so we let
         // them know.
         if let Some(old_tree_id) = old_tree_id {
-            if *old_tree_id != locked_wc.old_tree_id.to_legacy_tree_id() {
+            if *old_tree_id != locked_wc.old_tree_id {
                 locked_wc.discard();
                 return Err(CheckoutError::ConcurrentCheckout);
             }
diff --git a/lib/tests/test_working_copy_concurrent.rs b/lib/tests/test_working_copy_concurrent.rs
index 205e4f39e..490663973 100644
--- a/lib/tests/test_working_copy_concurrent.rs
+++ b/lib/tests/test_working_copy_concurrent.rs
@@ -33,27 +33,17 @@ fn test_concurrent_checkout() {
     let repo1 = test_workspace1.repo.clone();
     let workspace1_root = test_workspace1.workspace.workspace_root().clone();
 
-    let tree_id1 = testutils::create_random_tree(&repo1);
-    let tree_id2 = testutils::create_random_tree(&repo1);
-    let tree_id3 = testutils::create_random_tree(&repo1);
-    let tree1 = repo1
-        .store()
-        .get_tree(&RepoPath::root(), &tree_id1)
-        .unwrap();
-    let tree2 = repo1
-        .store()
-        .get_tree(&RepoPath::root(), &tree_id2)
-        .unwrap();
-    let tree3 = repo1
-        .store()
-        .get_tree(&RepoPath::root(), &tree_id3)
-        .unwrap();
+    let tree_id1 = MergedTreeId::Legacy(testutils::create_random_tree(&repo1));
+    let tree_id2 = MergedTreeId::Legacy(testutils::create_random_tree(&repo1));
+    let tree_id3 = MergedTreeId::Legacy(testutils::create_random_tree(&repo1));
+    let tree1 = repo1.store().get_root_tree(&tree_id1).unwrap();
+    let tree2 = repo1.store().get_root_tree(&tree_id2).unwrap();
+    let tree3 = repo1.store().get_root_tree(&tree_id3).unwrap();
 
     // Check out tree1
     let wc1 = test_workspace1.workspace.working_copy_mut();
     // The operation ID is not correct, but that doesn't matter for this test
-    wc1.check_out(repo1.op_id().clone(), None, &MergedTree::legacy(tree1))
-        .unwrap();
+    wc1.check_out(repo1.op_id().clone(), None, &tree1).unwrap();
 
     // Check out tree2 from another process (simulated by another workspace
     // instance)
@@ -61,20 +51,12 @@ fn test_concurrent_checkout() {
         Workspace::load(&settings, &workspace1_root, &StoreFactories::default()).unwrap();
     workspace2
         .working_copy_mut()
-        .check_out(
-            repo1.op_id().clone(),
-            Some(&tree_id1),
-            &MergedTree::legacy(tree2),
-        )
+        .check_out(repo1.op_id().clone(), Some(&tree_id1), &tree2)
         .unwrap();
 
     // Checking out another tree (via the first repo instance) should now fail.
     assert_matches!(
-        wc1.check_out(
-            repo1.op_id().clone(),
-            Some(&tree_id1),
-            &MergedTree::legacy(tree3)
-        ),
+        wc1.check_out(repo1.op_id().clone(), Some(&tree_id1), &tree3),
         Err(CheckoutError::ConcurrentCheckout)
     );
 
@@ -82,11 +64,7 @@ fn test_concurrent_checkout() {
     let workspace3 =
         Workspace::load(&settings, &workspace1_root, &StoreFactories::default()).unwrap();
     assert_eq!(
-        workspace3
-            .working_copy()
-            .current_tree_id()
-            .unwrap()
-            .to_legacy_tree_id(),
+        *workspace3.working_copy().current_tree_id().unwrap(),
         tree_id2
     );
 }