Not many here say that you can not do what you want. I believe it is the conversation that a good sign should not be altered. Weather you want to change YOUR sign to something that pleases YOU more is not the question I believe, it is the thought that someone is changing something strickly for financial benefit that is a good piece of history already.
It is the next buyer or buyer after that that may or may not know, and yes it is up to them to do their homework.

Craig