To get it to 'look' like an X6 file you can do a couple of things:
1. Import all f your X6 defaults into the plan,
2. Copy/ paste hold the entire plan into an empty X6 template plan. This is probably the best approach. A little housekeeping before you copy/paste the plan into the X6 template will make the process go smoother. Clean up the old plan layers, purge the materials, and any unused wall types. Set up the template plan with the correct floor/ceiling defaults to match the old plan and it should move over fairly well.