Do This At Your Own Risk.
It would be sort of a shame to do this with a nice Andrews holster but, you could dunk it in warm (not hot) water for a while.
Since the leather will already have some sort of protective final finish on it - you'll need to really let it soak for a while until it really absorbs the water.
If you have a lot of minerals in your tap water then soak it in room temperature distilled water. Just buy a gallon jug of distilled.
And since you are going to have to leave your firearm in a wet holster (possibly overnight) - don't trust a regular plastic bag!
Buy an Oven Cooking Bag since they are Nylon or Teflon and will not tear or rip like a cheap plastic bag...oil up your firearm really well and put it into the oven bag...insert it into the wet holster and start pressing the holster with your fingers to reshape, remold, and stretch it to your new firearm dimensions.
If the leather has absorbed water and the fibers get wet the holster will remold and reshape to some good degree.
Doing this may mess up the original final finish but, if you are wearing it IWB - then it should not matter what it looks like as long as it's functional.
NOTE that you'll probably have to leave your firearm in the holster at least until it begins to dry out and then carefully remove your gun and allow the holster to dry naturally all the way. Don't force dry it.
That should work OK since you can get away with a bit more with an IWB style holster than with an OWB.