Reposition expansion tank
Provided you aren't moving the expansion tank in relation to any other valves and pressure regulators moving it is fine.
The procedure for doing so is:


Determine the length of new pipe and number of sleeves you will need to move the tank. 


My guess is you will be making 2 cuts and thus need 2 sleeves provided the the pipe you are working on is a straight line (one down stream of the tank and one at the place you want to reposition the tank to)

Shut the water off up stream of the site you plan to work on.
Drain the pipes downstream of what you plan to work on.
Cut the pipes
Reposition the tank
Use the bicycle pump valve (Schrader valve) and a bike pump to inflate it to the appropriate pressure (hint: it should be the same pressure as the upstream pressure reducing valve)
Open your upstream and downstream valves to check for leaks and remove any air in the lines.