It depends on the author. Normally once paid, the game is yours. But some authors bring out new versions at higher prices, so you need to pay that price to own the latest updated version.
Like, you bought it at $5, but v2 is now at $10 - you won't be able to get V2 unless you pay $10... (paying the difference does not seem possible, unfortunately).
Other authors bring out the latest v2 at $15 and then after a certain time, drop down to the original $5, so you just need to wait.