This matter has to be interpreted in accordance with the Indian Contract Act. Let us examine this part by part:
1) Is offer letter an offer or Intimation of Acceptance :-
Fundamentally speaking there are two documents one is offer letter and the second is appointment letter / joining letter. In the case of an offer letter, where there are information about the salary or consideration along with an offer to join the company. But in case of an appointment letter the stage of offer and acceptance is complete and a final document is delivered. In the previous case their is an offer that you may or may not accept. In the later case you have a contract.
Things To Remember While Drafting A Valid Employment Contract
Contract = offer + accepatnce
2) In the case of acquisitions positions of issued offer letters or appointment letters :-
Any acquiring company takes the acquired company with all covenants attached. So all legal resources that are available against the acquired company are now available against the acquiring company, unless specifically excluded in the acquisition documents. So appointment issued before acquisitions are to be honored. But the conditions are precarious when it comes to offers, as they are not yet contacts. See second para of
Things To Remember While Drafting A Valid Employment Contract
So if they above issue is based on pure offers then the new company can revoke them before acceptance.