Arriva are getting new buses, it is now confirmed.

However, no E400s are transferring out of North East, as they are all earmarked for Euro 6 conversion for Newcastle CAZ.
The new buses coming into North East will instead withdraw life expired Volvo B7s and DB250's.

7511 and 7512 are probably also very likely to be withdrawn, as they will be 15 years old this year and I don't think Selby would want older buses than the ones they have now?
As mentioned 7513 doesn't exist.
7514-7517 will stay with North East and likely converted to Euro 6, if they haven't already. They still have a year or two of service left.