Looking at your previous questions, i figured you using WebSphere Portal.
The themes and skins developed for WP220.127.116.11 are backward compatible with WP6.1. If you dont want to use the sexy features of WP6.1 like drag and drop. Once the migration(using IBM supplied built in scripts) of WP 18.104.22.168 is done to 6.1; ideally/theoritically the existing code of 5.1 theme would work.
BTW you asking too many questions ;-) New to Portal !!! Read some books, may be the one written by Cameron.
Kishore Chekuru ( Kumar )