I remember when I changed from v19.07 to the v21.02.0-rc4 version, there were several changes automatically made to the settings. It worked, but I eventually changed back for reasons unrelated to this problem.
It is very important to save a config copy so you can go back to the v17.07 state, and if you are running OK, do the same for v21.02
I can't remember enough to be sure of the update changes that are made to the config, not even sure if 2 or 3 popup warnings appeared. There are quite a few changes to the structure and contents of the config files; some well-documented tweaks, bypassing LuCi, will no longer work because the files to be edited no longer exist.
It's worth being paranoid about config back-ups.