Any issues upgrading a specifically pre-partitioned fusion drive to an APFS MacOS?

I'm going to be helping someone soon with a MacOS upgrade (probably to Mojave) on a Retina iMac 27 from a handful of years ago that still has El Capitan installed, which is the OS it came with.

The only reason I even feel the need to ask if everything should be OK on this forum is because the iMac has a 3TB fusion drive, and that fusion drive was actually partitioned into two parts when the iMac was first purchased.

I am aware that APFS-required MacOSes (Mojave and Catalina) can be partitioned.

But I have not yet heard of this specific scenario.....the upgrading of a fusion drive that has ALREADY been partitioned as an OS Extended drive.

I would love to hear from others with direct knowledge of this scenario as opposed to any assumptions.

Should everything go fine?

If so, when the upgrade to Mojave is complete, will the separate partitions still be partitions, or is it possible that the upgrade-to-APFS process will make them into volumes when it's over?

Or is this actually going to be a real problem and the best way to do it is to backup and wipe the internal drive completely as well as turning it all back into just one single partition before installing Mojave from scratch?

Any good advice from experience would be greatly appreciated!

Thanks