I found two previous hints in previous threads & combiined them to help my family member regain access. It turned out that one of the main obstacles was he was trying to access his personal (secondary) My VM account to make changes. He actually needed to be in the primary/master account to do this. He hadn't used that since 2009 (!).
Anyway here are the posts that helped me - good luck 😀
1. from @Mingsy (https://community.virginmedia.com/t5/Managing-Your-Account-Cable/Re-Unable-to-change-sign-in-email-address/m-p/5532229#M261859)
I had a similar situation last week. The workaround steps are listed below but bear in mind that it didn't work first time but succeeded the following day.
- Log in with your 'old' email address.
- Go to where you change your email address. Use a made up email address and follow the instructions.
- While still on the page change your email address again to the one you want to use. The new confirmation email should then work.
2. from @Robert_P (https://community.virginmedia.com/t5/Email/Secondary-e-mail-account-locked-out-again/m-p/5532648#M276934)
Sign in to My Virgin Media
Go to Account settings, then Account details
Tap Manage next to Manage your Virgin Media Mail app password under Virgin Media Mail
Then, under Virgin Media Mail app password tap Get password
Continue the flow and a new secure password will be generated, and the mailbox unlocked
Update any third-party apps that may be use to access your Virgin Media Mail emails with this new app password.