1. Determine if the G21SF has an ambi-mag release or not. The G21SF can be had both ways. If it has the single-sided mag release, the old mags will work. If it has the ambi-mag release, it'll need the new style mags.
2. The new mags should be the only new ones available from now on.
3. If you do need the new style mags, with the window cut on the front, the old ones could be modified if you're good with a razor blade (Xacto) knife and can cut a window in the front of your old mags in exactly the right place. If you ruin one, you should still be able to use it in your G30 or just throw it away as it's only worth ~$20.
4. If it's a big deal, get a 21SF with the single-sided mag release.
(When I got my G21SF with ambi mag release, I only owned one old-style 21 mag. I cut a window in the front of my old mag and now it works in my G21SF and my G30.)
