I have it as an upgrade option.
Are you sure you are clicking through upgrade and not tariff change?
I have paid off the device just the tariff is currently running, i have option to cancel giving 30 day notice however was planning on changing the tariff to more data. Seen the 250gb deal as a sim only option but trying to get it as an upgrade but doesn't show. Would i have to cancel the existing tariff and take the deal out as a new purchase?
You can do that then just migrate your number to it Guide: Migration & porting into O2
Track usage, upgrade and get support with the My O2 appDownload My O2