I have been with Optus for close to 8 years with moble and interet.
Every time I have changed plans I have been charged a "month in advance" yet have never been credited with the originally paid "month in advance".
I have been told I only get those back once I leave Optus. Surely this is basically a rip off.
I suspect if you look at the bills you'll find you haven't paid anything extra.
Optus transition 1000's of accounts to new plans every week. Standard practice is to prorata any remaining amount for the current billing month back to you as a credit (so if you have 10 days to go on your old plan you get back one third of the monthly fee). You would then start a new billing cycle and owe the new plan amount (one month only).
This would all appear on the first bill and it looks very messy.