Originally Posted by
HerrKaLeun
You can use one of the HG 12-speed cassettes. the new microspline hyperglide cassettes and chains are just better, but not necessary. Manufacturers want to tell you you need everything from the groupset, but you don't. Same for crank and chainring that can come from a different manufacturer. I wish people would stop giving advice that sounds like you need to upgrade everything to get a specific derailleur.
My intention wasn't to imply that you have to have a matching groupset. OP sounds new to MTB drivetrains and specifically mentioned GX Eagle and new XTR 9100, neither of which are compatible with his/her current HG freehub. And to be clear, none of the new Shimano 12-speed cassettes will be compatible with HG freehubs- everything they've released so far (XTR 9100, XT 8100, and SLX 7100) will use micro spline for the cassette.
But yes, you could use a SRAM NX Eagle 12-speed cassette paired with Shimano 12-speed derailleur/shifter, and existing cranks with a new front chainring (as an example). Just thought that might add further confusion for the OP.