Removing is rather simple as well ...
Shut off the gas and disconnect the gas tubing going to the front of the valve... (Which if you have the original valve out you have already done).
Remove all of the screws around the outside edge where it meets the AS skin. Pull out the unit... you may have to pry it a away from the skin as the sealant may make it stick to it. On the inside you (may) need to disconnect the water inlet and outlet... but if you can slide the unit out just a bit it may give you access to those screws while it is still half way in... in which case you should also be able to put back in a replacement bracket (I'm assuming one came with the valve).
If you can't access the screws with it half way out then disconnect the water lines (shut off water valves and drain tank first...) the unit should slide out
There will still be wires attached so if you have a small table that you can slide the unit out on (and the wires are long enough) then you can get it out enough to access. Cut the wires if necessary but make sure the power to them is off (12V) if you have 110v that should have a plug.
Make sure you cut the wires about in the middle where it is easy to crimp them back together with an automotive wire crimp. Mark them if they are not easily identifiable by color.
Reverse the whole process after you have finished your alteration.
Piece of cake...