Can you read and write from and to APFS under High Sierra to FAT external hard-drives? Like many I have backups on external hard-drives, some are in HFS and others are in FAT32 or NTFS. 

What happens to those backups with APFS ? I won't be able to access, read and write to or from them if I update to High Sierra to get APFS ? Thanks !
What happens to those backups with APFS?


Nothing happens.

Only the main internal drive is converted to APFS (and its contents are preserved anyways).

You will be able to use your external drives as before.