This AutoCAD routine from Kent Cooper is pretty specialized, but if you can use it, it will undoubtedly save you a lot of time. It converts regular polygons to circles. What is a regular polygon, you ask? It's a polygon whose sides are all equal in length and whose vertices are equidistant from the center. Load the LISP file, then type in the command PG2C. Select the polygons and press Enter. Qualified entities in the selection set will be erased and a circle will be created in the place of each one, circumscribed about the vertices of the original polygon. The properties of the original polygon are also applied to the new circle. The LISP file also includes modification instructions to create an inscribed circle, and/or retain the polygon if this fits your needs.