diff --git a/mercurial/commands.py b/mercurial/commands.py
--- a/mercurial/commands.py
+++ b/mercurial/commands.py
@@ -2750,15 +2750,15 @@ def update(ui, repo, node=None, rev=None
     tip of the current branch if none is specified.
     See 'hg help dates' for a list of formats valid for -d/--date.
 
-    If there are no outstanding changes in the working directory and
-    there is a linear relationship between the current version and the
-    requested version, the result is the requested version.
-
-    To merge the working directory with another revision, use the
-    merge command.
-
-    By default, update will refuse to run if doing so would require
-    discarding local changes.
+    If there are no outstanding changes in the working directory, the
+    result is the requested version.
+
+    If the requested version is a descendant of the working directory
+    and there are outstanding changes, those changes will be merged
+    into the result.
+
+    By default, update will refuse to run if there are outstanding
+    changes and the update spans branches.
     """
     if rev and node:
         raise util.Abort(_("please specify just one revision"))
diff --git a/mercurial/merge.py b/mercurial/merge.py
--- a/mercurial/merge.py
+++ b/mercurial/merge.py
@@ -364,8 +364,11 @@ def update(repo, node, branchmerge, forc
                     raise util.Abort(_("there is nothing to merge, just use "
                                        "'hg update' or look at 'hg heads'"))
         elif not (overwrite or branchmerge):
-            raise util.Abort(_("update spans branches, use 'hg merge' "
-                               "or 'hg update -C' to lose changes"))
+            if wc.files() or wc.deleted():
+                raise util.Abort(_("update spans branches, use 'hg merge' "
+                                   "or 'hg update -C' to lose changes"))
+            # Allow jumping branches if there are no changes
+            overwrite = True
         if branchmerge and not forcemerge:
             if wc.files() or wc.deleted():
                 raise util.Abort(_("outstanding uncommitted changes"))
diff --git a/tests/test-merge5 b/tests/test-merge5
--- a/tests/test-merge5
+++ b/tests/test-merge5
@@ -12,9 +12,14 @@ hg commit -m"comment #1" -d "1000000 0"
 hg update 0
 rm b
 hg commit -A -m"comment #2" -d "1000000 0"
+mv a c
 # in theory, we shouldn't need the "-y" below, but it prevents
 # this test from hanging when "hg update" erroneously prompts the
 # user for "keep or delete"
+echo % should abort
+hg update -y 1
+mv c a
+echo % should succeed
 hg update -y 1
 
 exit 0
diff --git a/tests/test-merge5.out b/tests/test-merge5.out
--- a/tests/test-merge5.out
+++ b/tests/test-merge5.out
@@ -1,4 +1,7 @@
 1 files updated, 0 files merged, 0 files removed, 0 files unresolved
 removing b
 created new head
+% should abort
 abort: update spans branches, use 'hg merge' or 'hg update -C' to lose changes
+% should succeed
+1 files updated, 0 files merged, 0 files removed, 0 files unresolved