Do I have to get rid of my existing CMS to implement OpenScholar? Or can it get added slowly over time?

You do not have to get rid of your existing CMS. You can add OS incrementally, as many universities choose to do. Some start with only the new faculty sites. Others start with departments that are most in need of an upgrade. The beauty of OpenScholar is that the more that your university uses it, even via slow adoption, the more you will save on IT costs and the more your public facing sites will modernize.