Answered my own question; it is possible to convert a G21 to 10mm, and at less cost than I initially thought. Swap out the magazine, and get a new barrel and a heavier RSA and you're in business. So says these guys anyway. Hope this helps.
https://www.thetruthaboutguns.com/frankenglock-10mm-45-glock-21/
RSA is the same between the 20 and 21.