Hmmm... I've always let my cannas go dormant before dividing and moving them, but I actually can't think of any reason not to improve their lot now. It would be best if you waited until all blooming was finished, and only minimally divide the roots. The less the root system is disturbed, the less they will suffer from transplant shock.

If you are going to just lift them out of the pot and into similar soil, I'd say go for it now. If you want/need to divide the tubers, I'd say wait until fall.

Also, up here we are still in drought condition and the cannas in pots are doing ten times better than the cannas in ground.