I didn't get to choose. The only option was "Would you like to update to AFPS?"
Can you check in Disk Utility for me whether your Macintosh HD is case-sensitive APFS? Select the Macintosh HD disk there and then press the Info button on the toolbar. From your screenshot I infer that you had the regular HFS+ before.
In Sierra, Apple specifically labels case-insensitive APFS disks in Finder, like this:
Unless Apple changed the Finder labels in High Sierra, you should have a case-sensitive file system now.
Case-insensitive HFS+ to case-insensitive APFS for me. No change as to which is preferred.
Same as above. Did you check in Finder or Disk Utility?