Well, if their lease says "rent-free in perpetuity", then the landlord would have to buy them out.
Judges have ruled the gov't does have to keep its word. People have donated land and buildings for a specified use with the provision the donated property be returned if the use is changed and the gov't laughs at them. A woman donated a farm for a nature preserve here in Indiana and the state sold the land to "fund education about nature at other sites".