For example you could hire a Northern unit to work Leeds - Doncaster shuttles and flip some of the London - Leeds services to Edinburgh.
I’m not sure you could, unless some cross country or TPE servcies didn‘t make it to York from the south. And it would (obviously) be deeply unpopular in Yorkshire.
The other thing you could do is buy some of GC's slots back off them as these generally do line up OK with XC, and reality is by sending those services to Edinburgh instead of Sunderland you could easily make enough money to cover it. Again you'd probably have to pay GC enough to cover their costs running the York - Sunderland section.
That you could do, although it eouldcost a lot more than having to pay GC their costs.
All of the above comes with a lot of pain points and cost, but far less than the costs of trying to modify in 350m or 400m trains.
This is true.