Just use move directly to the subvolume main directory. For example. There is a subvolume called "backup" which you want to rename as "activeroot". Do:
mv backup activeroot
Be sure the subvolume is read-write
sudo btrfs send /backup | ssh root@othermachine "btrfs receive /mountpoint_other_btrfs"
watch -n 10 "sudo btrfs fi show"
watch -n 10 "sudo btrfs balance status /mountpoint"
watch -n 3 "sudo btrfs filesystem usage /mountpoint"