It's been difficult to locate a new XTR M9100 or XT M8100 with the SGS (long) cage from the few reputable U.S. dealers I buy from where Shimano will honor their warranty. The GS cage versions are readily available. In the past, I know it's been possible to switch cage length on Shimano derailleurs. In fact the Dealers Manual for the above derailleurs has a section describing replacement of the plates that make up the cage but doesn't specifically state its possible to change between GS & SGS cages. The key is whether the main body of the derailleur is different between an SGS cage der and the GS version.
Looking at the Exploded Views of the M8100 GS & SGS versions--they are separate documents, which is different from EV's of older model ders that had one exploded view that showed both GS and SGS cage assemblies on one document, inferring the same derailleur body is used--it appears all part numbers are the same except for the differing cage plates.
I have a source for replacement SGS cage plates for the M8100 and the GS versions of the entire derailleur are available and I wish to run a 1x system with a 10-51t rear cassette which requires the SGS version to be used, sums up my motivation to faff around with this, as the Brits might say.