I'm trying to make a simple mod where you can upgrade beehives with rarer and more expensive materials, such as iron and diamonds. I just started, both the mod and learning Forge. I've noticed the MAX_OCCUPANTS variable in BeehiveBlockEntity is final, so it cannot be changed, but I feel like that's quite arbitrary, so is there a way to make this possible?

MAX_OCCUPANTS is only used in (I think) two places, isFull and addOccupantWithPresetTicks (why the latter doesn't just call the former, I don't know). You could simply extend the class and override those two methods so your BlockEntity can look up how many occupants it is supposed to allow.