It'd be easier to do in MU format then it would to do it in MMPORG format.
But within MMPORG format...
You could basically keep the battle system constant with previous games. But it'd be hard to keep anything else the same.
The only way a Lunar game would work in MMORPG format is if they break the entire game down (in programming terms) like that of Guild Wars (every zone except cities is an instance) and they frequently have each major section of each major instance govern by a staff member of which applies a story line that does not contradict the over all world. Otherwise, it will just be a nice looking game of cyber walking and hack'n'slash.
Let's not forget that one of the key things that make Lunar really worth our time is the animation.
I agree with Angelalex242. You could do many good things with the battle system. However, that's about all you could do.