I'm thinking of upgrading to macOS High Sierra, however I'm a bit cautious with the APFS, also because it just came out.
Are you able to use Time Machine on a Mac with APFS-formatted disks with an Apple Time Capsule that uses an HDD an older file system (HFS+)
My guess is it won't work so you have to also manually convert the Apple Time Capsule to APFS.
Best Answer
Having an APFS-formatted system backed up to an HFS+ Time Machine/Time Capsule disk is actually the currently intended use case. The current version of Time Machine supports APFS only as a source disk, not as a destination. In fact, if you format your Time Machine backup disk to APFS, Time Machine will re-format it back to HFS+ before using that disk. This was discussed in WWDC 2016, and appears to still be true.
APFS does not support directory hardlinks, a feature of the filesystem used by Time Machine. Until Time Machine is rebuilt to take advantage of APFS features (like snapshots) instead of hard links, APFS won't be usable as a destination disk.
Also, I personally just backed up my APFS laptop to my usual HFS+ backup disk via Time Machine, so there is some empirical validation.