RAID Z Expansion Snapshots vs Balancing

I have a 4 drive RAID Z2 and I want to add a disk to it. As I understand it, this will not give me the 'full' 3 drives worth of capacity because the old files will take up about 25% more space than if the pool had been created with 5 drives from the start. The only way to get around that is to rewrite the files or more accurately, the blocks.

But then this would mess up older snapshots, right? Is there a feature for this? Or is one planned? I'm not in a hurry, I waited quite a while for RAID Z expansion.