[Tsiklon]

From my personal experience with the developer betas on my laptop;

I experienced no issue during the conversion of Corestorage Filevault to APFS Filevault. The conversion to APFS was offered as an option to me during the Developer Beta series.

I am not sure whether this will be done automatically for the release version. However I do understand that the conversion will only be offered to systems with Flash Storage only, whereas Fusion Drive and Hard Disk equipped systems will not. (as per: https://support.apple.com/en-us/HT208018 )

I found that the process required no interaction on my part and that my system came back up without issue. Likewise, there was no change with how Time Machine operated for me, in fact I don't believe that Time Machine backups are touched by the conversion process.

On another fusion drive equipped system I have, I was able to manually enable Filevault after filesystem conversion to APFS. You should be able to disable it after conversion too, though I've not yet tested this out.