The experts' rule-of-thumb is during dormant period, in the Fall or early Spring before they wake up.

Before I knew any better, I've moved a few beauties around (at less than ideal times) as long as I get the entire root ball & with a lot of TLC ... moved 2 ESs 3x, F&E 2x, 2 Serratas in late Spring... they were young & it was easy to dig up the entire ball as if I was just moving a huge pot from 1 spot to a better one (I was blest!) ... DON'T follow my ignorant lead, unless you won't mind losing you beauty.

I have an old Nikko I'm thinking to move, but fear the risk of losing 2 stems, on ground-layering status since 1st of June. If I toughen my chicken nature, I might still ... around mid-Nov ... & cross fingers/talking to Hydrangea gods all Winter!!! LOL

A very old shrub would be difficult to move w/out some damage to roots!

Experts will be responding soon! Good Luck!

I've had good luck with spring. Feel like the plant is primed and ready to go then...all you need to do is add the right environment. I've severely manhandled some hydrangeas in the spring with not a one lost.