The ADA does not have a provision to "grandfather" a facility but it does have a provision called “safe harbor” in the revised ADA regulations for businesses and state and local governments. A "safe harbor" means that you do not have to make modifications to elements in an existing building that comply with the 1991 Standards, even if the new 2010 Standards have different requirements for them. This provision is applied on an element-by-element basis. However, if you choose to alter elements that were in compliance with the 1991 Standards, the safe harbor no longer applies so the altered elements must comply with the 2010 ADA Standards.
A “safe harbor” does not apply to elements that were NOT addressed in the original 1991 Standards but ARE addressed in the 2010 ADA Standards. These elements include recreation facilities such as swimming pools, play areas, exercise machines, miniature golf facilities, and bowling alleys. On or after March 15, 2012, public accommodations must remove architectural barriers to these elements listed above are subject to the new requirements in the 2010 Standards when it is readily achievable to do so.