I thought the ITV licence or franchise meant it had to be a free to air channel
Certainly they could move the ITV2/3/4 to a subscription channel, but would the revenue from that outweight the advertising loss by reduced viewers. That possibly depends on current viewers.
However the advertising income days are probably numbered. With increasing channel choice viewers are spread more thinly. With increased use of PVRs viewers see less ads from the programmes they record (I watch less and less live now), and finally with streaming catch up type players the normal advert streams are bypassed.
I suspect our current TV model is going to change rapidly in the next few years, especially as the analogue to digital swithcover completes.