You can "thin out" Cytisus right now but don't cut too hard back and definitely not into old wood.

They hate transplanting and especially if yours is big enough to need pruning I just wouldn't bother. Just replace with another plant, bought or your own from cuttings taken right now. Just use this seasons shoots no more than 6ins in length in gritty compost around the edge of a terracotta pot with poly bag over in good light but not direct sun.