Not being an expert on Tank manufacturing, I would imagine the plant would still operate for the purpose of repairing the current tanks the Army already has. Since these new tanks are not the new generation I would assume they have not re-tooled yet. This would mean they could just re-tool prior to the start of manufacturing the new generation tank when the time comes.
Again, just a guess on my part.
Allow me to provide some insight, based on my experience with program oversight.
First, manufacturing a piece of equipment is done by a Primary Contractor in their own production facilities, while depot level maintenance is
usually done by the service at one of their own depots. So, No, they plant would not stay open if production is discontinued. This is simply a "jobs program" move by Congress, especially the Congressmen from the district / state the plant is in. Unfortunately, this means the money the Army wanted to use elsewhere on other things is now directed by Congress to be used on making unwanted tanks. It happens, all the time.
However, once production is complete, the primary contractor usually shifts production towards something else. The tooling used is usually just scrapped, unless the military pays to store it (which also is very expensive). And remaking that tooling to continue production later, or tooling for something new, is also hugely expensive; lust look at the cost a car manufacturer faces when they start a new car line. We're talking BILLIONS.
I do applaud the individual Congressmen for looking out for their constituents. But it amazes me that there isn't some check / balance in place that prevents the interest of the very few from overtaking the interests of the very many. Part of the "you vote for my pet project and I'll vote for your's" mentality in Congress, and it ends up costing the Nation....