[squirrel_hunter]

If you're swapping tanks wouldn't it just be easier to remove the tank from the bike, open the filler cap and tip the contents into a tub? With the tank upside down you can then remove the pump that you'll need for the new tank anyway and turn the tank the right way up to get the rest out. Give the tank a shake to get the last out as there are internal baffles in it, and you're done.

Saves trying to kill your battery and the tank and pump needs to be swapped anyway so helps with that. Only advice left is to have some rags handy to catch the extra spill, the pump will retain some fuel, use a new o-ring on the pump, and spray a little WD40 on the cap as the petrol would have given it a wash would be a good idea.

This is how I have swapped tanks before.