Can a script be set to say change ignore combat if an actors health is low ?

I don't believe so; however, you could probably add conditions to either separate procedure branches within a single combat override package, or separate packages within an actor's combat override package stack, to cause behavior changes based on health.

I didn't think so either. Besides, I suppose you want this to be quick and a package change with a evaluatepackage is faster than a procedure change. So, the easiest and faster way make as Dreamking says, using a different package in the same combat override list.

Thanks