The only time you would lose eligibility to upgrade @Anonymous would be if your line was disconnected from the network entirely due to arrears. In this case, you're not eligible for upgrade until 6 months after you reconnect, as a positive payment history needs to be established again. This is something which is controlled by the system, and is also covered by the T&C's as well as explained by the Payment Management team at the point of reconnection.
If you're late with a payment, even if this results in a restricted service for a period of time (althgouh, crucially, not if it results in disconnection - see above) then the usual requirement is for the account to be brought fully back up to date, and be free of arrears and restrictions for at least 24 hours - you'd then be eligible to upgrade as normal.
Was your line disconnected? Or were you simply restricted due to an overdue bill?
I work for O2, however any advice or opinion given in this community should be considered my own and not necessarily representative of Telefónica UK Limited.
