Surely, whether it's logical or not depends on a number of things, not solely what the existing paint scheme is.
If the current white tails are due off lease anytime soon, or if Stobart have another intended use for them then it makes no sense to paint them.
Equally, if EI have reduced their requirement from Stobart then there are likely restrictions on Stobart operating an EI liveried aircraft on a non EI flight. There may be a grace period but I doubt it's very long. You can only imagine the effect on the EI brand in the event of an incident involving an EI liveried aircraft when it wasn't even operating an EI flight.