I think that's extremely unlikely, as Northern will have entered into a lease contract with Porterbrook for 319s until the end of the franchise. This is a legal contract between two companies and if its terms are breached by either Northern (not paying for the trains) or Porterbrook (not supplying them) then that party will be in court. The only circumstances I can think of where Porterbrook might release them from that contract are:
- If Northern wanted to lease something more modern and expensive from Porterbrook, this increasing Porterbrook's total income (and Northern's costs). The 360s are owned by Angel and it's next to impossible that Porterbrook would voluntarily give up their income in favour of a competitor.
- If Porterbrook had someone else who wanted to hire the 319s and would pay more for them, and Northern was willing to let them go. The only way 319s have a unique selling point is if they are converted into 769s, and there's no sign that the demand for them will mop up the currently off-lease 319s let alone those already running in electric mode for Northern.
- If the 319s are so unreliable that they fail to meet any availability criteria that may exist in the lease. Even if these clauses exist, the 319s now seem tolerably reliable so it's unlikely they would be invoked.
It's remotely possible that there is a break clause in the contract, which would allow either party to terminate it at a specific time, but if so Porterbrook would most likely charge a risk premium. As Northern can deliver their franchise commitments with 319s there is no real upside for them in agreeing to this on the off-chance something else will come along whose benefits justify the cost and hassle of changing fleets.
Probably the only applicable situation would be if Northern got some external funding to increase its electric services. If they are additional then the existing contract would not be broken and the 319s would not be "binned". Even then, unless the service uplift was improbably large, Northern would most likely go for more 319s instead of a microfleet of something else.