change version numbers inside processing function as well

.github/update-release-branch.py

@@ -175,7 +175,7 @@ def get_today_string():
   today = datetime.datetime.today()
   return '{:%d %b %Y}'.format(today)
 
-def process_changelog_for_backports(target_version):
+def process_changelog_for_backports(source_branch_major_version, target_branch_major_version):
 
   # changelog entries use a speficic format to indicate
   # that they only apply to newer versions
@@ -191,6 +191,7 @@ def process_changelog_for_backports(target_version):
 
       output += line + '\n'
       if line.startswith('## '):
+        line = line.replace(f'## {source_branch_major_version}', f'## {target_branch_major_version}')
         # we have found the first section, so now handle things differently
         break
 
@@ -206,10 +207,11 @@ def process_changelog_for_backports(target_version):
       # filter out changenote entries that apply only to newer versions
       match = regex.search(line)
       if match:
-        if int(target_version) < int(match.group(1)):
+        if int(target_branch_major_version) < int(match.group(1)):
           continue
 
       if line.startswith('## '):
+        line = line.replace(f'## {source_branch_major_version}', f'## {target_branch_major_version}')
         if found_content == False:
           # we have found two headings in a row, so we need to add the placeholder message.
           output += 'No user facing changes.\n'
@@ -375,10 +377,7 @@ def main():
 
     # Migrate the changelog notes from vLatest version numbers to vOlder version numbers
     print(f'Migrating changelog notes from v{source_branch_major_version} to v{target_branch_major_version}')
-    subprocess.check_output(['sed', '-i', f's/^## {source_branch_major_version}\./## {target_branch_major_version}./g', 'CHANGELOG.md'])
+    process_changelog_for_backports(source_branch_major_version, target_branch_major_version)
-
-    # process changelog for backport to target release branch
-    process_changelog_for_backports(target_branch_major_version)
 
     # Amend the commit generated by `npm version` to update the CHANGELOG
     run_git('add', 'CHANGELOG.md')