i154: are roles invariant?

This is a follow up of the discussion that just occurred at the
f2f.

Consider the message below. A simple inspection of the message is
enough for a node to determine it should process blocks <X> and
<PlayTheFollowingRole>, and only these two blocks. Processing
then starts. However, during the evaluation of
<PlayTheFollowingRole>, the node also discovers that it must play
the role "meAsWell". If it assumes this new role, it breaks the
invariant (on roles - i154); if it does not, it breaks the
contract represented by <PlayTheFollowingRole>.
  <envelope>
    <header>
      <X actor="next" mU="true">...</Y>
      <PlayTheFollowingRole actor="next" mU="true">meAsWell</X>
      <Y actor="meAsWell" mU="true">...</Z>
    </header>
  </envelope>

Is it important for us to support such changing roles?

Jean-Jacques.