I would hope that the Brum-Euston remain as 350 for that reason. Allows unit swaps much better.With the plan supposedly being thus:
Euston-Tring/MKC: 730
Euston-Brum: 350
Euston-Crewe: 730
Brum-Liv: 350/730 depending on who you ask
...by service groups I meant those - i.e. are you going to have a problem keeping 730s and 350s apart if there is interworking between unit diagrams of Euston-Brum and other services.
The Euston Crewe will not be a problem as that's worked by the 2 main depots for the 730/2 anyway.
The Liverpool service I have concerns with, though diagrams already can support only 730 at my depot, that won't be the case for Wolves/New St.