Can Chicago condo association be forced to hire a union janitor?

We have a single employee, a janitor whose retiring and is part of a union.

We want to hire a non union janitor. Again, our ONLY employee is a janitor who happens to be in the union. We're told we HAVE to hire a union janitor FOREVER. Not just that we HAVE to have an employee forever, but it will always HAVE to be a union one.

Is this true?