We cut ours down by 6 feet last year. Re-grown beautifully.

If you mean to lower it there's no problem. If you mean to make it narrower I'd wait a bit longer. We narrowed part of ours in August (the drive was getting too narrow!)and it is beginning to grow next years leaves - not good at this time of year. We knew it was too early but it really needed doing...

we took the one on our boundary down by 20 ft to 12ft. it didn't seem to bat and eyelid [I know they don't have them but you know what I mean :o) ] we did it early spring whilst it was still dormant and we could see where we were cutting.