I'll take this question in general, not specific to Hams Fan's army.
If the paintjob is OK and not too thick, I would never remove the paint. You could paint over one or two colours to fit your new colour scheme.
This is because priming and basecoating is a taxing job, and redoing those phases will spend your time on something you had ready in the first place. Also if you have based your figurines before priming, as many do, the paint removal will destroy that job too.
But if the figurines are really badly and/or really thickly painted, then paint removal is required just to bring the sculpts back to life. Still, on a couple of occasions, I've kept even a bad paintjob and improved it the best I can - on rank & file figurines. Figurines that are looked at individually require better paintjobs.
The only situation where I often remove paint from decently painted figurines is when I'm planning to sell them. If the work on priming, basecoating and basing has become unimportant to you, paint removal is easy and people tend to prefer unpainted figurines. Then again because paint removal from metal is so easy, people are usually ready to do it on their own, so paint removal isn't that important to get metal figurines sold. Plastic may not sell at all if badly painted. But the time spent on removing paint from plastic - which is not always easy - may be much more valuable than the worth of the plastic.
Eventually, everything boils down to personal values on time & paint quality. These vary wildly. So do what you feel is best.
-Z
