I'm sure alternative calls at Manea and Whittlesey could be arranged? (Though maybe not - it appears that some routes are going to have better services on Sunday than during the week, for reasons that are not clear but suggest bureaucracy ruling!)
I think very almost all of GAs new trains have now been delivered (and the rest are surely in such a late stage of production that cancelling them wouldn't be worthwhile)? If something were to be cascaded to replace older trains somewhere else then I can't imagine any other part of the country wants the unique (and seemingly quirky) Stadlers, so that would mean that there could be some Aventras (which are / will be used in other parts of the country) spare?