I know of a diocese that is set to buy an old Proddy church to be used by its indult parish, so things work both ways.
This is not uncommon. Our Maronite Catholic parish purchased a boarded up Methodist/Episcopal Church which we are now in the process of renovating. Once it is completed, the bishop will come up here to consecretate and rededicate it as a Catholic Church. Father originally approached the RC Diocese, hoping to purchase one of their closed churches but was turned down.
Our Maronite Eparchy's Cathedral is also in a former protestant church. They did a beautiful job in its conversion.
"I know of a diocese that is set to buy an old Proddy church to be used by its indult parish, so things work both ways."
It's actually fairly common, especially with eastern Catholic churches. New construction is very expensive, so purchasing an existing church is often the way to go. For whatever reason, it seems to be more common for eastern parishes to purchase Protestant church buildings than western (Roman) Catholic ones.