@MD-Benefits Guy you are correct to ask questions because the when and how the acquisition is done can have a significant impact on your current plan's participants.
First and foremost, pay attention to @david rigby's comment about whether the buyer will acquire all of the stock of your company (the seller) or the buyer will acquire all of the assets of your company (the seller). If this is a stock transaction, then upon closing the buyer in control of the plan. If this is an asset transaction, then upon closing the buyer is not in control of the plan and the seller continues to exist after closing and the seller can decide the fate of the plan.
You do not say if the buyer has an existing 401(k) plan or, if not, intends to adopt a 401(k) plan. If yes, there are rules about whether the buyer's 401(k) plan is considered a successor plan to a seller's plan that is terminated after closing, and these rules can be particularly onerous after a stock transaction. The more common scenario for handling an acquired seller's plan is for the seller's plan to be merged into the buyer's plan. A plan merger is different from a plan termination.
If the buyer expects to have an existing 401(k) plan operating alongside the seller's plan, each plan's document should be carefully reviewed to address potential unintended consequences. Each plan's provisions regarding eligibility, excludable employees, plan compensation, contributions (including answering your question about true-ups), vesting, and the safe harbor features should state clearly what applies to all or each subgroup of employees. This review definitely should be done before closing a stock transaction.
Another note to keep in mind is that plans are terminated by adopting an amendment to terminate the plan. In addition to setting the termination date and the plan year end date, the termination amendment can be used to address the questions you raised in your original post.
Mergers and acquisitions, handled properly, can go smoothly with few surprises. Handled improperly, they can lead to bad feelings and costly compliance issues. Hopefully, both the buyer and seller in this transaction have experience with what needs to be done with existing plans of both the buyer and seller.