This is how I have done it:
1. Add to workspace and open older revision
2. Save as backup
3. Remove from workspace and erase not displayed
4. open backup file and save to workspace make sure the file name matches the file in Windchill
5. Search Windchill Commonspace for part and checkout the file when checking out make sure "Reuse modified workspace content" is checked
6. Check the file back in to Windchill. The old revision will now be saved as the latest iteration.
If you don't need A.5 and A.6 you could just delete them and then you would be back to A.4 but, if there are references it could be difficult to delete them.