Can you use an APFS-formatted Mac with a pre-APFS Apple Time Capsule? 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.
Yes, you can use a normal Time Capsule that is not APFS-formatted with APFS-formatted disks.


  APFS and Time Machine
  
  You don't need to change any Time Machine settings to back up
  APFS-formatted disks. Any Time Machine share points must be shared
  over SMB instead of AFP.


Source: Apple.com

This applies to Apple’s TimeCapsules as well. They are just network attached hard drives with Wi-Fi routers made by Apple.