Do not know what problems you might encounter going that direction, but I transfered a couple dozen P6.5 Mac files to the PC and they worked fine in CS2. Now, remember that PP 1.5 was not available for Mac. Did not come back to that platform, until CS3. Could be some little glitch there, but *usually* older versions' .prproj files open in the new versions. However, they *usually* do not go the other direction.
If you have a Matrox card, or similar, or a lot of 3rd party plug-ins, these could cause more problems.