Upgrade '22 FEX's SLX Shifter to XT

It sounds like you have the i-Spec SLX brake lever, so the shifter you have listed will work with it perfectly. You'll simply unscrew your existing shifter and screw the new one on, just like you said. As far as changing the cable, you'll actually completely remove the cable with the shifter. The housing (housing is the standard name for what youre calling the sheath) will stay in-place, no need to remove that. New shifters come with cables so then you can thread the new cable through the housing and re-adjust the rear derailleur. Good luck buddy, dont be afraid to ask questions. Its a pretty simple process. The new shifter should also come with a couple cable ends, once you get the cable cut, make sure you crimp on on so your cable does fray on you. Oh, and dont cut your cable too short, its normal to leave it an inch long or so.