So like, if I buy FFIV for GBA and then they release a DS version, I have to buy the DS version?!

If you're wanting to play it on a different platform, then you pay again. SE originally released XIV on PC, so 2.0 is free on PC since it's the game we should have got originally, paid for when we bought XIV.

Inb4 QQ: when XIV came out, we were told that PS3 version was "coming soon". If, hypothetically, the PS3 release of 1.0 worked out, would you expect to pay for it again? Yes. This is no different.