They were new drives for me so I was reformatting anyway.
But Google says you can:
https://www.quora.com/How-can-I-convert-an-external-HDD-to-APFS-without-erasing-its-contents-on-Mac
There's an option to convert in Disk Utility if the drive is HFS+.
If I just plug in my old HFS+drive it seems to keep working normally. Maybe it doesnt benefit from AFS.
Also not sure if I should start a new TimeMachine or keep my old HFS+ one?
Thanks!