If you wish to move it as one plant, the trick is to get as large a root ball as possible.
This is easier said than done as I move three very old ones and had to dig huge deep holes to try to get as much as possible.
Two of the three are doing OK although I think I have bad spot as they are not getting as big as they should be after a fair number of years.
The third blooms every year but is not really getting bigger and I am going to move it this fall.
From my experience it seems once they are truly established depth means little but if they are too deep when you replant, they just seem not grow and not die.
IF they are too high, you can always put more dirt on them but if they are too low it is not fun.
