Similarly Microsoft offered FAT32 to NTFS in-place migration, and it did the required checks before starting to ensure it completes successfully. It was more than 20 years ago IIRC.

The notable thing with APFS is that the migration came automatically with an OS update (to high sierra) by default.

Yes, I think also mounting ext3 systems as ext4 and migrating it to ext4 as you access it is a good way too. IIRC you had to enable a couple of flags in the ext3 system with tune2fs to enable ext4 features as well, but it didn't break, and migrated as you write & delete files.