Solved! Solved: Go to Solution.
I've read up on the FAQ's for the Apple Music Promo.
Just for other customers who already have an existing Apple Music sub billed for directly by Apple and are concerned that they may lose their song library.
Apple Music song library alert
Customers who already have an Apple subscription must select to continue with Optus during the activation process. This may generate an alert suggesting they will lose their song library.
However, this is not correct.
Glad to hear Apple Support were able to help out with this one @LONGTERMUSERDWR