When a major buys a junior then not only a shaft and shovel is bought but the complete deal, assets and liabilities. That includes all contracts and agreements.
Those contracts do not change when the name of the junior is replaced by the major. The major is obligated by a contract as before.

Simply put, the major operates as the junior and in many cases the junior's name is still in force and only becomes a subsidiary of the major.

I am not a lawyer so take my opinion for what ever it may be worth.

Ed.