i'd go with Chris' suggestion, but make sure you keep it well watered as large hydrangea's in my experience don't like to be moved. last year i planted a lovely pink mophead i'd grown for 5 years in a pot to the front border.

It didn't like the move to say the least and at least half the stems just gave up over winter and produced no shoots. last year it was covered in masses of large flowerheads, and this year only two are just starting to open. It gre very slowly, and only in last month or two has it started to look healthy again. Next year i expect (hope) it will produce new stems from the roots, and 3 years will be back to its best again

just give it tlc and i'm sure it will recover from the move in time