I have a pionsetta plant that I've had for over 10 years. It is now the end of May and it's still blooming. Can I cut it back and repot it while it's still blooming? Or would this kill it?
There is no reason to repot it. Normally, it is pruned back in January, but it can be pruned way back at any time. The purpose of the pruning is to keep the plant full and compact and to promote colored bracts next holiday season on the new growth. You are a bit late on the pruning and that may mean you won't get the colorful bracts at the end of the year. But pruning will not kill the plant.
You should repot while it is in a non blooming state.