You will always be able to download the latest version. FYI though the price will increase at some point and I don't know if itch will simply give people who paid $5 the game or if I increase the price to $10 will people need to pay an additional $5.
Either way you won't lose out too much paying now.
FYI though next update will be a very different game. Current release was based on the previous donation release rather than current development path.
Check some of the blog updates on the itch page to see more about that.