There is a process in changing an expansion tank and having that valve doesnt matter when done right.
If you shut that valve now and try to change your tank, you will have 12-20# of pressurized hot water spraying into your face.
I've been doing this full time since 1978, that is my qualification
I never have and never will put a shutoff on an expansion tank, it's a recipe for disaster.
In this case, snug up the packing nut and be done. When you do get to changing the tank, do away with the shutoff.