The D'Addario website has a string tension chart. If you look up the string gauge you've got on now and find it's tension tuned to E, then find the gauge that is at or near that figure when tuned to C you'll have maintained the appropriate balance across the instrument. Don't however, then tune the new string up to E or D, for you will find that you have spectacular levels of neck relief at best, the bridge in your lap at worst. Hope that helps