you wont have problems loading any Sonar X1 projects but you may want to take note of the plug ins which you used in X1 and if you used V Vocal on those tracks, its not included in platinum. Melodyne is the new Pitch correction software and its quite a bit more in-depth and better than V Vocal in my opinion.
You will not have access to Sonar X2 or X3 by upgrading but you wont need them as Jesse mentioned.
those versions are rolled in to Platinum and Platinum is far superior than those previous versions.