Less shock than you would expect. I purchase my JM's throughout the season, in person and usually in full leaf. I bare root at that time and replant in a better quality potting mix (all of mine are grown in containers) and I make any necessary root corrections at that time. It may set them back some but not a full growing season.

Regular maintenance root pruning and repotting I try to do when the trees are fully dormant but with my collection, that doesn't always happen either:-))

You must have better luck that I do. Last year, I did the same root prune on one that arrived fully leafed out. It was just 1 root snipped. It was not happy I did that. Some leaves sorta dropped early. The ones remained just hung out all season and dropped early in Fall. It didn't die but no summer flush on it as well.