I see many guides online which start with a new plant. This is what mine looks like and it's been here for years, so I'm not sure if i can guide it into a tree anymore. If so, i guess now that it started budding would be a good time to trim everything but the leader?
I think it would be too much effort to try and train an old shrub into a tree when hydrangeas are so insanely easy to take cuttings of, and the cuttings grow so fast. (So much so that when we prune our hydrangeas many of the fallen branches accidentally become new plants).
tho I love hydrangea bush's too and this is a lovely mature one. So I reackon you should keep this one, take a cutting, and grow the cutting into a tree somewhere else, and then you get the best of both worlds (: