It'll cost yer !
That said, my understanding is that once airborne, the aircraft effectively trims itself, and therefore seat changes should be available - there may be a requirement to return to allocated seats for landing however.
Please note that from 1st January 2009 an extra charge of 5 pounds/euros will be levied per passenger on all Ryanair flights on each movement of seat forward (towards the sharp end) and a deduction of 2 pounds/eurs per passenger on each movement rearwards (towards the blunt end).
RaF