Get a razor saw, like this - the wider blade
Then start cutting under the base of the foot, parallel with the metal base. Make sure you're not cutting upwards (you'll lose the foot), or downwards (you'll end up cutting too much base). Gets pretty easy once you get the hang of it.
Yes, you'll lose a little of the foot, but careful cutting and rebasing deals with that pretty easily.
For very small feet - and I'm thinking like the Spider - you can also use a Stanley knife blade. Take it easy, rock it back & forth. It'll cut pewter - lead very easily - and you don't tend to lose material. But it's only feasible for smaller cuts.
The toes may bend during the process - that's OK, bend'em back later.
