Thank you for replying :-)
"Photos 5.0 should be able to open a Photos 3.0 Library directly". I will try this tonight and provide an update to this thread.
"But you will no longer be able to open the Library in Photos 3.0 on a different Mac after you opened it in Photos 5.0". True, probably holds true for the same Mac as well. This lends itself to the lack of backwards capability, sadly. I noticed this when I was converting iPhoto files to Photos: those files then became "migrated". I don't know if I could open them up (again) in Photos, but I will hold onto them in the case that I lose the updated files.
"Are the Photos 3.0 files on an external volume"? In fact, yes they are. I will copy some to my MBP and see if they will open. I'll provide an update on this as well.
"Photos 5.0 cannot work with a library, if the external volume has been used by Time Machine". That would make sense. I copy these files on their own onto an external HDD. I then drag & drop them onto my MBP as needed.
"You need to put the files on volume, that is formatted MacOS Extended (Journaled) or APFS and not used for Time Machine backups". True! I have my current MBP (mid-2015) under 10.15 which is in APFS format.
Thank you for your feedback! I will provide an update on this thread.
Cheers!
-Merlin