Thief class to Ninja job would make sense mechanically: Both are suited to dual wielding short blades. The thief class could have Evasion enhancing skills while the ninja job could add enmity generating skills to that.

As for a DD class becoming a tank job; well, we already have similar with a DD class (ACN) becoming a Healer job (SCH). Ideally, though, THF would have a second, DD, job also (assassin?)

Lore-wise, it's still up in the air.

FF purists would probably balk at Ninja being a tank again, though. But it's no more sacrilege than the almighty SMN becoming a DOT job.