Why would you do that? Legal issues in your state (i.e., no 15-round mags allowed) and you've got access to some 15-round mags?
Unless you do the conversions out of state, you aren't bypassing the legal issues, as you'll possess them in their illegal form and you can be burned! Law enforcement types might not know that they've been converted, and you'll still have problems, waste time, have expenses, etc., etc.
Just get some new mags or try to work a swap, out of state, by exchanging your 15 rounders for some 10/12 rounders.