Your 4G grandparents were born in the 1700s. With people who lived that long ago, you are not going to upset another descendant who has fond personal memories of the ancestor and doesn't want their profile to be sullied.
You should, however, be respectful of their work. In the profile text, describe the data you found on the profile and indicate why you think it was wrong and what your sources were for the information you replaced it with. For example, you might say that the profile showed a marriage date of 1782, but cited no source, so you replaced it with the marriage date of 1774 that is documented in the vital records of Anytown.