I did the same thing when I was putting my TTI extension on a 40 rounder. I thought I was stuck with it, because I couldn't force the extension off enough to push the spring down.
Then I did this: bent a little hook in a wire coat hanger. Slipped it down beside the follower as far as I could reach into the mag, hooked the spring, and pulled it back towards the follower. It moved the spring enough that the TTI base pad slid right off. Done.
Of course this only made sense after much energy was expended. And of course cussing and beating the mag on things.