[Stooky Bill]

The mux which contains (most) ITV and C4 stations would need re-encoding but in theory it could be done as the current DVB-T mux has 13 SD channels and if switched to DVB-T2 it could probably hold 6 HD and 6 SD channels.   Think it's far more complicated than just flicking a switch but it could be done if there was a will to do it - sadly that doesn't seem to be the case despite the obvious benefits.

Switching the MUX to DVB-T2 should be fairly straight forward technically, Arqiva recently refreshed their encoding and multiplexing so it will be compatible. However..... 

There's three broadcasters on that MUX, two of them own it. They'd all have to agree to the change and obviously work out which channels they'd lose. Secondly it would mean that anyone without a DVBS-T2 receiver would lose all the channels on it 

However as the BBC seem to now be planning to scale back to one mux and ditch their SD simulcast that will hopefully have a knock on effect.

 
I'm not sure there's any plans for them to do that yet, nothing I've heard of. Ditching their DVB-S broadcasts is a much bigger deal for the BBC of course, as has been seen with the changeover on satellite