You won't be able to connect to a shared drive using AFP anymore, if you get the new file system APFS when you upgrade. Apparently you only get APFS if you have a pure SSD drive (not Fusion).
https://support.apple.com/en-gb/HT208018
APFS and file sharing
- Volumes formatted as APFS can't offer share points over the network using AFP.
- APFS supports SMB and NFS, with the option to enforce only SMB-encrypted share points.