Errrrrrrr. They can. If TfW have a binding contract with start date then the trains have to transfer regardless if Greater Anglia have any replacements. If the 170s stay longer with GA then TfW would have to agree to it. See the cascade of 156s, 158s and 170s from ScotRail to Northern where ScotRail have had to backfill with unrefurbished HST sets as an example.