I've just done the same on an old Avon, replaced the A4's with C7's.
New valves supplied with ready made doubler patches from Ribshop. They do a kit with most things you need, glue etc.
In addition you'll need a heat gun to warm up the old patch, it'll then peel off quite easily. The hole that's left needs to be opened up slightly larger to accommodate the new valve. You might want some kind of power tool to remove all the old glue and bits of valve that get left behind, the little piece of sandpaper that comes in the kit will require patience and elbow grease, not supplied