I dun think they would go about moving spells from one class to another. People who played CON to get elemental spells would be upset
When SE asked "Will you welcome drastic changes for the greater good of the game?" a majority answered "yes."
It's like asking an American if they believe in freedom. Most will say yes.
Then ask them which political party best exemplifies that freedom... eh... not so much agreement anymore...
I really don't know which way this could go.
BLM to CON - If you follow the Gridania storyline, home of the Conjurer's guild, you know they're all about communing with the Elementals that dwell in their forest. It's a big, big deal and central to the whole **** plot. As the role of BLM is traditionally one of dealing damage with elemental magic as far as Final Fantasy goes back, it seems like a logical choice. However...
BLM to THM - Thaumaturge is a dark job. Sure it can help others, but only if you inflict wounds upon yourself to get it done. BLM would forgo its traditional elemental role, but it would do so in exchange for an even darker existence.
WHM to CON - Conjurer has spells traditionally reserved for WHM. You'll find all the classics. Cure. Raise. Protect. Shell. White mage would seem perfectly at home here.
WHM to THM - Thaumaturge may be a dark, emo class, but WHM is no stranger to self-sacrifice. Even in more recent changes to FFXI, WHM can be empowered by the pain that's inflicted to it or even by taking on the pain of others. The Thaumaturge guild in Ul'Dah also has "forbidden magics" that could possibly even raise the dead... Sounds a little familiar, does it not?