Convert external HD from OS extended to APFS without losing data

While it can be done under certain circumstances, this is a risky procedure and should never be done unless you have a full backup of all your data. Even then, imho, it would still be preferable to wipe the drive and reformat as APFS from scratch, then copy your data back to the newly formatted drive.

Disk Utility can do that, but why?

This article has instructions, and the pros and cons of using APFS on a hard disk drive.

https://www.ifixit.com/Guide/How+to+convert+your+Mac's+storage+from+HFS+to+APFS+file+system/150453

I don’t think you need to go into the Partition tab, but I’m not a Mac to check. I think if you select the drive in the disk list, there is a convert to APFS command in one of the menus, maybe Edit.