It's probably best to close up shop completely, and re-open as a different entity.
If ABC Construction has a labor agreement with Local 123, and then decides to not renew the contract .... well, expect all hell to break loose.
First off, the employees have a RIGHT to join a union. You don't have any say in the matter - all you can do is negotiate in bad faith (also not allowed), and look out your window at the picket line. Open your mail, and you get to see your lawyer's bill for answering the NLRB complaint.
Open as a new firm, different employees, and you're good to go. The new employees decide to join the union, you get to start all over again.
Another alternative is to 'organize' the employees yourself, and have THEM "decertify" the union. This has happened, but is not easily done. It would be especially difficult in our 'craft' union setting, as the union also manages the benefits.