An England spot is not an entitlement, but being told the truth usually is. Dylan Hartley would quite clearly be Montpellier-bound if he didn't think it would adversely affect his world cup. If they do then turn around and pick French-bound players, they have effectively lied to him and every other EQP. I believe Lancaster's quotes in public over the last couple of years back this up and shoot down the matter of it being flexible, although I haven't looked at the matter in a while and can't claim to have seen all of them.
Ultimately, if Lancaster is perceived as having been untruthful with his players, I don't see that going down too well. I don't think they are going to complain, or should complain, if at the last Lancaster makes a decision within the framework they know that results in a few different players. But suddenly departing from that is going to be a very different kettle of fish.