Grandfathering usually applies only to stuff that's already in place at the time the new policy is passed. If you decide to prohibit wet-over-dry renovations, pre-existing installations are grandfathered in. If you want to ban pets, you can't force people to get rid of pets they already own, but you can stop them from getting new ones. If subletting is no longer permitted, current subletters may remain until the end of their subleases, but no longer. An amorphous desire to sublet at some time in the future wouldn't be grandfathered, even to people who bought their apartments with sublet possibilities in mind.

It's a fundamental principle of co-ops that House Rules are subject to change at any time, and the Proprietary Lease may be amended by a supermajority vote of the shareholders. Everyone who buys into a co-op agrees to these terms as a condition of purchase. Policies change -- sometimes for the worse -- but as long as these policies are administered even-handedly, shareholders are bound to obey them.

(I can't help but think of people who moved to the United States around 1900, enthusiastically noting that income taxes were prohibited by the U.S. Constitution....)