A hub can do that, but if a unit (transaction) has more than 1 mutation its invalid. Only when 1 witness is replaced by others as well, when the new witness has posted enough stamps, the "stability point" can be moved forward, and then the next witness replaced.
In other words, if you replace one witness with yourself and I replace another witness with myself we'll split the network into two halves. There will be 50% of users behind your choice and 50% of users behind my choice. And of course, I will never accept you as a witness and you will never accept me. Looks like the consensus will diverge for indefinite period of time in this scenario.
the one allowed mutation is sufficiently small compared with the 11 unchanged witnesses. In case a user thinks that any of the witnesses has lost his credibility, or there are just better candidates, the user can replace the witness with a new witness in his list, bearing in mind that his witness list may not differ from that of other units by more than one position. This means that any changes can happen only gradually, and a general consensus is required for a change bigger than one position.
So end users should not be allowed to change more than 1 witness on the default list or they would be separated from the rest of the network. Obviously this sound too like a problem. Can we have some clarification here ?