Is there any situation in which this can be deemed acceptable? Shouldn't the current contract be extended to give the Contracting Office the opportunity to either reissue the Solicitation or to at least give the competitors the opportunity to re-bid? It should be noted this is Trade Off and not LPTA. How can this situation be rectified after the new contract starts?
In order to provide a more specific answer; many additional facts would be required (e.g. contract type, the extent of communications with industry before and after solicitation release, etc., etc.).
In order for the contract to be changed/modified the contracting officer would be required to do a scope determination. Things that are looked at during a scope determination include:
1) Is the change authorized by a contract term and condition, a clause? etc. If either yes or no... go to step 2.
2) Is it within the general scope of the contract:
a) Could the change have been reasonably contemplated at the time of award? If yes go to b)
b) If known, would it have altered the field of original competition?
If no, then the change can be made. If yes, then the new effort would have to be recompeted or the modification could be done if the contracting officer gets a justification and approval for other than full and open competition.
Again, in order to answer these questions a much, much more detailed knowledge of all the facts would need to be known!