Can I move up Fish Buffet levels?

Yes - absolutely!

If you earn the required Fish Points in the required time frame you will progress to the next level!

Example: You earn the 20,000 FPs at Plankton level before the 90 days, you immediately move up to Goldfsh, where you will have a further 90 days to earn the 20,000 to maintain or 60,000 points to climb to Shrimp.