I'm not saying the barrels you already bought won't work. But the barrels on the SF models are not the same. That is, unless they started using the newer barrels that came out on the SF models, on the new non SF guns also.
In other words, I compared my older Gen3 G21 barrel to my SF G21 barrel and they were different in the lug area. However, they may have started using that new design in all of the their guns. So if you purchase a current production G20, it may have the new SF style barrel, which would make them the same.