Ok if they have a stock shortage but the last time I checked there is some suitable stock stored at Ely in the form of ex FGW HSTs why not make use of these in the interim.
Different interior (seat reservation headache), different interior fixtures and fittings (maintenance issues and crew familiarisation required) and different kitchen equipment (crew training required plus complicates delivery of catering) off the top of my head. Plus are the vehicles there actually spare (i.e. not waiting for work at Wabtec) and if so are there enough vehicles of the right type to form a full 2+9 set?