On the subject of multiple transponder codes; units operating services outside controlled airspace are assigned a block of codes each; they will issue you with one of these to A) identify you and B) indicate to other units who it is you are talking to.
As regards being asked your full details when transferring to the next unit; your flight plan will only go to the relevant ATCC(s) and your destination, it will not be passed to en-route units outside CAS unless you specifically ask for it to be addressed to them.