Hey there.
As far as I understand how it works, if you have bought v0.6.3a on itch, you should be able to download v0.6.4a on itch too.
However I have already encountered "bugs". Like, if you bought it at 4$ during a sale, it won't give you access to the game when it's at "full price". It only happened once so far but it did happen.
Does that match your situation ?
Or maybe am I completely wrong about how Itch works ?
Thanks for the very prompt reply. The option to download 6.4a is greyed out, so presumably whatever I paid to get 6.3a wasn't enough to future releases as well. I purchased it in November last year. Guess I'll wait till you complete the game and pay one last fee. Great game, so well worth paying for it.