What I like to do to stretch leather holsters is cover them in boot oil, and then wrap the gun that goes in the holster in wax paper so that there is extra material to make it stretch. The idea is to make it VERY tight. Then I'll leave it for several hours, or maybe all day or all night. Then, come back, take the wax paper off, test fit it, and redo the procedure if needed.
The strap itself I would simply cut it and stitch in new material. A half inch stretch on a piece that small could prove impossible.
This here is the most extreme holster conversion I've ever done. I took a high quality flap .38 revolver holster, and converted it into a well fitting holster for my Sig.
I cut the flap into the shape of a strap, I cut the strap in half , added in some of the left over leather from the flap by punching holes in it and tying parachute cord strands through the holes, then super gluing it and wrapping it with Hockey tape. Then, I cut the seam out from the section where you see all the bolts. Then I punched the holes for the bolts, put the bolts in, oiled it up, put the wax papered gun in, and tightened the bolts down real tight. Then I let it sit. It reshaped nicely, but it definitely wasn't going to work that tight, so I loosened the bolts, and filled the gap with shoe goo.
Granted that's a much more extreme conversion than most people will feel like doing, but it serves to prove that your holster is in no way shape or form hosed if you don't want it to be.