Machining a 9000 series inner chainring to fit on a 9100 series crankset
Here's the difference in the chainrings.




Shimano might have modified one attachment point to ensure that the ring is oriented correctly. There does not seem to be any differences in the teeth though. 

As Shimano removed some material, it's obvious that you can. It would be possible to simply mark out the profile of the modified arm, hand file away the extra material and test fit as you get close to the line. Filing with reasonable precision is not as easy as it looks but there are many videos online that show you how it's done.

If you need to get someone to do it for you, look up local machining shops.