I'm sure this has been discussed before, but I haven't done an extensive search for the answer - the current IEP order provides 596 carriages for East Coast and Great Western to presumably replace the class 41 hauled trains, with Wikipedia making reference to further IEP orders for East Coast, which I'm assuming will replace the class 91 hauled trains - assuming that the 91's would be about 35 years old by then, is there a potential move to the (by then) newly electrified Midland Mainline (even perhaps some CrossCountry routes) as an interim step until new more suitable electric stock can be sourced be possible ?
