I think they can renege on the contract because there has been a supervening event, namely promotion to the Premier League and a requirement to provide more away fans' tickets.
On your other point though, I agree that it would be much better if away fans were moved. I think the Lower Brian Clough is too good for them though. I would put them in the corner of the main stand that borders the Trent End- F block I think it is, where the view is not all that good, where there is a degree of geographical separation from the Trent End due to the gap at the corner and where, and this is the reason for moving them, the influence of away fans on the game is minimised. It would also be nearer to the city centre for them, and not have them traipsing across the car park causing problems by the club shop. It is the sort of place that away fans are accommodated in at Crystal Palace, where, as many will know, there is effective home support behind both goals.