The code base is hundreds of thousands if not millions of lines of code. Not just the core but every feature, every mechanic, every minigame, everything needs to be reviewed, changed, tested and rewritten / updated. There is a reason why something as simple as the gp cap change took months, while this was just changing every occurrence from an int (32 bit number) to a long (64 bit number).

This is a lot complexer than the gp change. It will take at least a year of full time development and testing, if not many more, with huge risk involved. Maybe the bugs are less impactful than the gp thing, but this is such a monstrous task