It's probably likely there isn't tech available to make such a transition. Changing a character model can have pretty far reaching consequences if you didn't design your game around having that as an option from the start. There also isn't a system for microtransactions set up for FFXI, so they'd have to set that up too.

While it's a good idea, and I'd love to have the option myself, it would be a big project and the dev team has shrunk to a skeleton crew. I don't think we'll see big features like this any time soon, unfortunately.