You can use the two hole saw with a hand drill, as mentioned.
One hole saw fits into the other on the arbor. The one is the diameter of your existing hole, the second one is the diameter of your new hole. Piece of cake.
I'll look for the pictures I have posted here on CN before.
Double saws a.jpg
Double saws c.jpg
That's what I was going to suggest, but I've never tried it so I didn't know if the threaded portion of the arbor is long enough for two hole saws. Now I know. Thanks.
epenna, a hole saw with an arbor is $20 or less at Home Depot, and a hole saw without an arbor is about $10. The arrangement Garyth64 suggests will cut a nice round hole for your new focuser, centered where your old focuser is. It will make for a very tidy installation. The hole saws will cut anything your scope tube is likely made of (steel, aluminum, Sonotube, etc.).
It's counter-intuitive, but I suggest using your drill at its highest speed if you use a hole saw. High RPMs and light pressure (so you're almost filing with the hole saw rather than cutting) will give you a clean hole and minimize the chances of the hole saw "grabbing" and tearing into your OTA.
Oh, and your paper template idea is excellent for locating the screw holes for your new focuser. Of course, you could also simply hold your new focuser in the hole and mark the screw locations with a felt-tip pen or pencil (if you've put tape on the OTA first).