Tuning down a twelve string is common and recommend, both from a tonal standpoint, improved playability, and imo most importantly, the health of the instrument. Pretty much every twelve string acoustic guitar gets a bulging top to some degree, and many have bridge and bridge plate issues as well. That's a whole lot of string tension trying to rip apart some very thin pieces of wood. Something has to give, so dropping the tuning down a half step or more is pretty much standard operating procedure.