Hi iComm
When you see the message 'This file has been upgraded for use with a newer MYOB product' the bottom left of that message will show an 'r' number. What is this r number?
Premier v12.0 is r12.4, Premier 12.5 is r12.5
You would see this message if your file was r12.5 but you were running Premier 12.0, not the other way around. It is likely the file you are trying to open is r13.5 (which means you will need Premier v19), or the Premier you have installed in v12.0, open the sample file, even if read only and see Help > About Premier - check the Aust release version.
Post your findings.
Regards
Gavin