Here is a site with some pruning tips from a couple of very experienced Hydrangea growers, Kristin & David VanHoose at HydrangeasPlus. I have used this method and it works very well. However, I did not move any of mine. Sorry to say that I mistakenly purchased a Sister Theresa from a local highly recommended source and it was a dud. Nurtured it for several years and it never grew larger or produced any blooms. It is now compost somewhere in the garden. A different replacement cultivar, from HydrangeasPlus was planted in the same location and it is now a very large shrub, covered in blooms each year. BTW, 10 other Hydrangeas, a total of 11 of 12, purchased from that same local source, wound up in the compost heap. Either, they died or never grew larger and never bloomed. Rb Here is a link that might be useful: Pruning Tips

I don't think there would be any particular problem because of the time of year. In fact it might be a good time. Assuming you can really dig the soil enough to get something besides chunks of icy soil. I don't know how frozen your ground really is. Water it very very carefully and keep it very well watered. I wouldn't take it inside. Hay

I think now is a good time (or at least tomorrow when it is daylight).

This reminds me of a very funny story . When I was 17 , The father of a long time friend replanted their hydrangea from the back of the house to the front. This man was well known to like his beer , wine ect... more often than not He was feeling no pain . When the hydrangea was in the backyard it was a blue color. After moving it to the front there were no blooms one year but the following year it bloomed a stunning pink! He was so elated and marveled at the color change not knowing at the time about soil PH and color change. The folks whom he told that were not aware of PH levels changing flower colors just assumed his story of how his flowers changed from blue to pink as being a by product of too much booze. It wasn't until I learned from working at a garden center shortly there after that I was able to tell Him why his Hydrangea changed color. We had a good laugh .