Mi 9 / Cepheus
Is a big jump from MIUI 12.1 - 20.8.27 weekly to this version possible or do I have to completely reinstall (format data)?
With the current version, switching from WiFi to mobile data no longer works, I have to reboot every time.
Going from Weekly to Weekly you should not need to format data, or anything else.
The important consideration is your data. How you do this would really depend upon how important it is to keep your data.
1 - Data is irrelevant. No format data should be necessary. Install the new ROM (over the top) and then do a Factory reset.
2 - Data is important. Back up what apps and data you want with something like Titanium backup, and back that up to your PC. The safest way is probably at each version change in the ROM, ie. Miui or Android, install the new ROM. Before each ROM installation, however, also tidy up the memory; clear Recents, etc.
If you use Magisk, or anything similar, avoid installing that until the end, and not until you are happy that everything is funcitoning correctly.
If you definitely want to keep your data, then you could do a full nandroid (eg. TWRP) backup before installing the ROM, each time. It is time consuming, but more secure.
These are the two ways that I would approach it, but, of course, it is completely down to you.
[Edit: You can choose your recovery app from here (Cepheus).]
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