It IS a completely different airplane, The original (Still one of my all-time favorites) is designed around the pattern planes of the early 80's while the new PLUS version is suited more to today's 3-D style planes. Plus, many of the differences are not only cosmetic, but structural. The new US is built light and with larger control surfaces.

It would be like bashing a Cub into a Citabria.

Don't bother, fix your Ultra Sport and have a ball with it!