You will always be a Founder or Co-Founder and your title will change. Be sure to be careful however how you dole out the Founder/Co-Founder titles. That should be a lifetime title so be sure it goes to the right people who played a major role in the starting of the company and who will continue to play a role in the years to come. Another option is to give yourself the title President if you have plans to find someone to run your business (say if you are a technical founder)