Upgrading

[OriginalDuffer]

How long after my contract has ended do I have to upgrade to a new device?

[jonsie · Accepted solution]

Your contract carries on month by month until you end it. You can upgrade at any time as long as your airtime is continuing.

[MI5]

@OriginalDuffer 

As long as you want.

There is no time restriction.

[gmarkj]

You don't have to upgrade to a new device if you are happy with the one that you have @OriginalDuffer.
What I would suggest is that if you originally took the contract out with a 3rd party, such as Carphone Warehouse, is that you consider moving to a SIM only deal.
If you did use a 3rd party and you have completed your minimum term then you will still be paying for the device as well as tariff and you could save money.
If you went straight through o2 then you would have finished paying the device off, and are now only paying for your tariff. Again you might be worth checking as it might be that you can get a better deal now.
But there is no requirement to upgrade to a new phone if you do not need one.