Can't convert to APFS in Mojave

You should be able to convert to APFS from HFS+ non-destructively through macOS Recovery - if your Mac has the required recovery partition. The usual method to boot to recovery is to hold down ⌘-R immediately after a power-up or reboot. Disk Utility will then become available through the Recovery mode GUI.

This site claims that the conversion will be non-destructive when done through Recovery mode. It covers High Sierra as the latest macOS because APFS was introduced with High Sierra and this was germane to users back then. I cannot find anything to contradict this assertion.

Your milage may vary.