APFS on Internal HDD

FWIW, it appears that is a 5400 RPM drive which would be in a 21.5" iMac. It's a slow drive to begin with so it could be slower formatted as APFS. You'd be better off with an external SSD and install macOS on it and boot from it. It would be much faster than the internal drive.

GiorgiApkhazava wrote:

High Sierra 10.13 works fine ... But i plan to go Catalina/Monterey - thats why im asking...

You're running a system before Catalina so you'll have to replace all of your 32 bit apps with their 64 bit counterparts or an alternative 64 bit app.

If that's the case download and run the free GO64 app to see his of your apps will not run in a 64 bit environment.

It should work the same way. You don't need to erase and install, just apply the new system over the top of the existing system. The user data will not be affected. However, good practice means you'll have a backup of the drive before applying any upgrade or update.