How about putting another bit of plastic/shopping bag etc on the inside so any excess glue goes on that. I've never tried it though, only ever used self-adhesive patches.

tear aid on the inside!

yes some old bladder will do the trick.

Thanks for the tips!

Hot water soak will remove all brands using glued valves. Soaking for a few mins in water just off the boil is the go.
If that doesnt allow you to peel off the valve then it will be one of the newer kites using welded valves.
Only option then is the cut-out leaving the bigger hole.
We use a piece of tissue paper inside the bladder to prevent the new valve sticking to other side problem.
Using a piece of bladder or any plastic sheet is not a good idea.
You have to be sure the film you insert doesnt seal to the valve blocking the airflow.
Tissue is ideal. Blows apart as soon as you pump. Newspaper or baking paper etc all OK options.
If you do use bladder or plastic punch a hole through it for the airflow.