Account

[Bob259]

If I upgrade to new phone, still got outstanding on old device what happens to the outstanding? Does ot go on new account

[PandJ2020]

If you perform a 'swap' then you return your old phone to Sky.  Hopefully the residual value will clear the outstanding balance.  (Any shortfall is payable in full on the next bill)

If you don't return the device (i.e. just buy a new device) then you'll pay for two until the balance is cleared on each.