I have been with Optus for about 10years, give or take a couple of years. Up until recently being migrated to nbn from cable broadband I can't complain about the service till now.
When I got notified over the phone by some sales rep from Optus saying that I had to move over to the nbn as Optus were going to disconnect the cable broadband service, in my area soon. So I did. This was around April/May. They neglected to mention that my Fetch TV service had to migrated over to nbn, so when I put in a technical issue that I no longer could recieve Fetch TV I had to wait till I got the new Fetch TV box to get service. That issue is now resolved.
Now just over the weekend my email account that I have used for many years is disconnected without any warning or notification. I have other email accounts that I use as well, but the Optus email I've had is important as it's the one I use the most. At first it I thought nbn service has gone down so I reset the modem on the Saturday just gone. Waited for service, nothing. Did the same thing on Sunday, still nothing. Then the Outlook app I use on my mobile asks for my password for my Optus email account. After many failed attempts to login I deleted the account from the app and tried to login when adding email account, still I couldn't login. So I rang Optus mobile Technical issues told them my delema, on the Sunday, on Sunday someone said at Optus that they will fix the problem. By Monday, two days ago, still no email. So put in another call to Optus complaining about it until someone from Optus said that the email account had to be migrated from cable broadband to nbn. It was not until Tuesday, yesterday, that I could FINALLY login to my email account. I asked also can all my previous email from the old disconnect account be retrieved onto my new email account as I had old important emails on there which I wanted to keep and I was expecting emails over the weekend that I need to get. I was told yes, and still nothing this morning. So I had to waste another morning, now being Wednesday, on the phone to get my old emails to be migrated over to the nbn email account. At the time of writing this message about the service, still no migration of old emails.
Optus Sales and Customer Service reps should let the customer know that the Fetch TV and email account will be disconnected from their current cable broadband service before switching over to the nbn. Customers should not have to go through these technical issues when the nbn service is not even up to scratch.
If my emails have not migrated over by the end of the day i might think about changing service providers.
The issues you describe are when the processing of the NBN change over are not handled correctly. So this isn't a "old to new" error, this is someone not doing their job properly and migrating the services correctly.