I wouldn't say fully wipe out gil but put a cap on how much can be transfered. Many people will cry and complain but in the end they tough it out. Reason why I suggest this is even though gil is still easy to get it is nowhere as easy as it was at launch. Majority of those who have constantly played since launch have well over 500m gil just hovering in their pockets. When a rush of new players come in (given 2.0 is recieved well) the economy would be too unstable and high for anyone to afford anything, swaying their opinion against this game from the moment they log in.

As for the other data, leave it as is. Levels and other data will not negatively impact the game and it's player s on the scale as the gil overflow would.

Edit: Oh and by cap I mean just transfer (not a real figure) 10m gil per player or something like that no matter how much they have stored.