Should old threads be left alone ?

It depends on the situation really. If the user bumping it has actually something to contribute or if something new that's relevant to what the thread is about has been shown in the comics, then why not? No need to create a new thread if an older thread about that topic already exists.

On the other hand, if the user doing the bumping, just does this to increase his post count or just repeats stuff that has been been posted in that thread, then no.

If it ain't locked...

Battles will remain battles.

I have no problem. But sometimes the characters got more feats or upgrades after the thread was made.

Yes.

If their not relevant to any current discussion or if you aren't trying to remake it, yeah.