How do I get the feet off?  

I haven't done it myself, but
From what I understand, if the foot is broken off, the remaining piece would need to be pushed *into* the leg so that a replacement glide can be installed "on top of" the old one.

I was wondering how you were...
I was wondering how you were gonna do that. I haven't been able to either.

have you tried gaspipe...
have you tried gaspipe pliers and some force? if that doesn't work i'd pull out the bottom part and drill through. you can cut it in 2 pieces through the hole and remove the thing.

They are a pain for sure....
They are a pain for sure. The real trouble is if you are trying to get a good one out of a bad or unwanted base like the stacking ones that nobody wants. I have gone at it several ways. If i am reclaiming a good one and the base is unwanted I cut the leg with a hacksaw and push it out. They have a metal part that is shaped in such a way as to sort of lock in the leg, so they don't pull very well. If I want the base they sometimes (not often) will pull out with the exact right size of needle nose pliers, but more often they snap the foot off. If you have a bad foot and want to put a new one in, the suggestion of pushing the remnant of the old one in is the way to go.

Olive, to answer you more...
Olive, to answer you more directly, they are one way designs. People do replace them, but they were only designed to go in and stay in, not come out. Think of a barb on a fish hook. they kinda work on that idea.

Drill it out
I've replaced nylon glides with new ones, I've never tried to get a good glide out of a leg (seems impossible). To remove an old broken glide, I take a drill and use a bit smaller than the opening in the leg. It's pretty easy as once you drill out most of the glide, the rest you can pull out with needle nose pliers. Sometimes the heat generated by the drill bit will help the remaining nylon stick to the bit so it comes right out with the bit when you back out. Wait for it to cool, then twist it off. All gone.

I gave up!
I saved a few bases for whitespike and myself that had 4 good glides on them, and I sold the rest. I was able to sell all 28 bases via craigslist. The woman who took them is going to repurpose them into a table or a scuplture or something. I'm happy they found a home!