Hi Sam: I'm assuming you are speaking about the front sight post and not the entire sight base that is pressed on to the barrel. The post is pressed into a dovetail. There is a tool to press it out. I used to remove the post by using a punch and knocking it out. It's in there pretty tight but with the judicious use of force and patience it'll come out.
I think the sight base is actually soldered on. So you would need heat and some type of device to pull the sight base off. Getting it back on aligned properly is another issue.
I can't imagine that the rust got under the sight base. But you never know I suppose. Let me check on something. I think I may have a tool to pull the sight base off. Next time i'm in the shop I'll check. It's a front sight puller for an M1 carbine but if I remember I think it works on the K31 too.