Number one tip when changing operating system: back up all of your data.
I have 3 tested external backups.
I randomly want to promote a very useful program, that I use every day:
That program is rsync.
More info here: https://rsync.samba.org/
Rsync is unique to other file copying programs, in that it can preserve ownership/permissions/flags and it can do backups incrementally - it can be run locally, or even copy over scp/ssh.
Rsync is your friend. Read the manuals:
https://rsync.samba.org/documentation.html