Sanctity of contract

Dear Editor,
As I understand it, for a contract to be legally enforceable it must satisfy the six elements of (a) offer (b) acceptance (c) consideration (d) intentions (e) legality and (f) capacity.
As a layman, I would not delve into the implication of each element except to say that once the contract fulfils these requirements, even if it is lopsided in favour of one party, it cannot be changed without the consent of the other. In other words, a unilateral change will make the contact void.
As an example, in an employment contract the employer cannot impose a change unfavourable to the employee without his consent. So if the employer decides to demote the employee, the employee can refuse the demotion and sue for constructive dismissal.
I am aware that a contract was signed between the previous Government and Exxon that some considered lopsided in favour of Exxon and are strenuously advocating for the current Government to re-negotiate the contract.
So, let us assume that the Government did approach Exxon for a change but Exxon refused on the basis that the terms in the contract are legally binding, can the Government then give Exxon an ultimatum to either re-negotiate or pack up and leave?

D Sookdeo