They could also do a re-balancing of the stuff you carry over. Like level 50 in Part 2 means you start with stats representing the cap in place for that game (which would result in overall lower stats). You keep everything you had, but their effect is slightly different to go in hand with whatever tweaks they'll have in place for the second iteration of the battle system. I don't know, that's how I'd like them to do it, personally, but we'll see how they handle it in 2-3 years, or whenever they start giving relevant info about Part2.
That's more or less how I think they'll handle it. Personally I think they may do it a bit like Mass Effect. If you played the previous game(s) you effectively got a starting boost for ME2/3 but still had to grow your party properly.
So if you're level 50 you might start on level 5 for part 2 and have a level 60 cap. Convert items collected to a lower but upgraded number of items (e.g. 99 Potions to 10 Hi-potions). Keep all your money but increase costs to purchase.
Maybe keep all your materia but it gets set back to a lower level with scope to further upgrade than in part 1. Or introduce Yuffie very early in part 2 and have her steal all your materia at the beginning so you have to start from scratch!