What rights does the founder of a non-profit have if he leaves the organization?
Like for-profit corporations, the focus is the business--or in your case, the people or causes being served. Unless there is a culture dedicated to preserving the legacy, there often is nothing in the law that says that the founder must continue to be recognized as such.
As a matter of marketing, however, it probably would be good form to recognize the substantial dedication that the founder brought to the organization in getting it started and running it so successfully over the years. It could make the difference between the friends of the founder continuing to support the organization or finding another worthy cause to contribute to.