With usage, old laser rods eventually will need maintenance by cleaning and refurbishing it. Does anyone know how this is done?
Well, rather obviously you need to find an optics house which will take on the task of regrinding the ends to optical flatness while maintaining perpendicularity of the faces to the long axis. Cleaning the barrel is easier, assuming there aren't any nicks or cracks. If the latter, it's more than likely that the rod will not survive much longer in any case due to the thermal stresses involved.
Depending on the application, you then follow up by having new coatings (AR or possibly an end-mirror) applied. This is a job that only a specialized optics coating house can perform.