My USB 3.0 drive that has a full install (not a Live USB) had a
/homepartition. I assigned the
/18.3 partition as the new 19
/partition, and chose Format the partition. Then I assigned the
/homebut without the Format option.
All seemed to go well, but 19 would have glitches here and there. Programs wouldn't open, some file permissions were incorrect, and some other things I can't recall.
I ended up doing a complete reinstall of 19 Xfce (which included formatting the
/homepartition as well) onto the USB. Working like a charm now.
Data backup wasn't an issue. I use this USB as my "portable" Mint.