Probably more practical from a cost standpoint to convert to 40, assuming a person shoots and keeps 40 on hand for other guns. There is also a 9mm barrel made (but it doesn't show up in the regular catalog) but I have heard mixed reviews about reliability using 9mm ammo in 10mm mags. If I want to shoot 9, I just take a 9 to the range.
Here's a good article regarding shooting 40 ammo through a regular G20,
http://www.thetruthaboutguns.com/2015/06/jeremy-s/40-in-a-10mm-glock/ 
