Migrate dataset snapshots from old pool?

I use zfs to maintain automatic snapshots of my files. I "upgraded" the drives last year by getting new drives and making a new pool. At that time I merely rsync'd all files over from the old pool, keeping dataset names the same. My thought was, if I needed to access the old snapshots, I could always plug in the old pool in an external enclosure.

But now I want to repurpose the drives that contain the old pool. How do I migrate the snapshots to the new pool, preferably without taking a lot of space? I do not use dedup.