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This is one of those things that if you know how to do it.... it's actually pretty easy -- but if you don't know how to do it, it's super confusing.
FF2 uses MMC1 (aka mapper 1). FF2 also currently uses 256K of PRG, which is the maximum PRG allowed for a typical MMC1 cartridge. However, there are board variants like SUROM that still use MMC1 and hijack CHR bits to allow for larger PRG.
So you have two options:
1) Switch to the SUROM variant
2) Switch to another mapper entirely. MMC3 (aka mapper 4) would be the next logical choice.
The good news is, both of these things are possible. The bad news is BOTH will require some significant code rewrites, as even SUROM requires different code from a typical MMC1 to swap. Not only that, but FF2 is probably tight on space in the fixed bank -- which is where the swapping code will have to be, so you won't have a lot of space to work with when writing swapping routines.
And even once you get the mapper changed, you'd need to actually cut into the text reading code and have it swap to a new bank... which may be incredibly easy or somewhat tricky depending on how the game works.
This is definitely an asm hack. You'll need to know 6502 basics and how to use a debugger and/or how to read a disassembly. If interested I can probably throw out a few pointers as to where to start, but I've kind of rambled enough already, and I'm not sure how much (if any) knowledge you have with 6502 and NES architecture.