Certified GKH Gardening Expert
Once you pull it out of container to plant and loosen the roots, the issue will be resolved. You can score the sides of the rootmass and pull the bottom roots out if they are circling the lower container. Be sure if it is going into another container there is plenty of soil all around it for the roots to grow into. However, when moving up in container size, only go up one or two sizes to prevent overwatering.
This article will help you with the care of hydrangeas: https://www.gardeningknowhow.com/ornamental/shrubs/hydrangea/growing-hydrangeas-hydrangea-care-guide.htm