Sorry to hear the gestapo are still alive and well. Those unauthorised hydrangeas can be so dangerous! Luckily everyone has survived them for the last 2 years, but I guess they'll have to go to your deck now.
Anyway, I hope the transfer goes well. I've moved a few plants from beds to pots, mostly with success. Is it well established? If so, it should be okay to move.
spring or Autumn are apparently the best times to move hydrangeas. You need to dig a large root ball, and place in the pot immediately. It's a good idea to have someone to help you lift it in if possible, as it may hurt your back otherwise. Also, water well straight away. I think it's a good idea to prune the plant before moving it.