Thats strange, Ive never ever had an issue changing the titles of my existing GTC listings that Ive sold some of (some Ive sold hundreds of).  Never received any message either.

Of course previous buyers would see a listing as it was when they purchased - you cant change a contract AFTER someone signs it. i.e. you cant change a listing after a sale is made, for the person who made the purchase. You can only amend for the others who are yet to buy.