I just had a really enjoyable software experience. Without explaining how I got into this situation, let me just explain the starting conditions.

I have a 1TB nvme disk in my system and had two btrfs partitions, / and /home. On this same system I have a handful of other partitions to enable dual-booting. The layout was something like this:

[[ntfs      512GB].....................[/home 64GB][/    64GB]]

And I wanted a layout more like this:

[[ntfs      512GB].....................[/home 128GB][/    64GB]]

So I create a new 128GB partition to land the contents of /home on and the magic happened:

btrfs replace start /dev/$old-home-device /dev/$new-home-device /home

And in an instant my /home filesystem was moved and the old filesystem was disabled. I involuntarily yelped with joy when I had realized that the process compeleted so quickly.

The final bit is to resize the filesystem to fit the entire partition:

btrfs filesystem resize max /home