I read your post as "I changed ownership and was told no fee", so I'm only reading what the OP wrote.
Given this post has changed direction multiple times and everyone says "Optus is wrong", best bet for the OP is to follow the escalation process Optus has publicly available.
www.optus.com.au/complaints and log a formal complaint since he belives he was "intentionally and egregiously mislead". They will review all the notes, chat logs etc on the account and determine if the claim the OP makes is valid or not. If it's a valid claim, then they will give him what he wants "a free upgrade" or perhaps even "contract release for another provider". If they cannot find any evidence to substantiate the claim, then they will look at how they can best resolve it.
The OPs post specifically asked about contractual arrangements.
Could someone please explain to me why this upgrade fee is nowhere to be found in any paperwork?
But you're correct this issue did revolve more around the POS representations (which was addressed if you read through all the posts). It can definitely take a few goes to get to the nub of an issue though.
In the end though Optus made good on the deal as verbally agreed and it looks like a definite win for speaking up.