Every 4-5 years it is good to give your herb garden a make-over, You can remove tired plants, add young replacements, re-organize the look and just get something new in there. Early spring is really the best time to do this. It is less stress on your plants.
Check out my vegetable gardening blog: The Rusted Garden. It is filled with garden information, videos, pictures, seed catalogs and seeds & things I sell. http://www.therustedgarden.blogspot.com