I'm pretty sure the way Itch works, once you paid it you own it, including future updates.
(Though I'm not the dev obviously, so don't necessarily take my word for it.)
I saw an Itch post that a moderator responded to and they said that if you buy a game and then the price increases you retain access to it and future updates without having to pay more money.
Yeah I deff think that's how it should work
