ok.. i'm currently riding a 12-25 shimano ultegra cog in the rear and a standard 53T in the front. my question is.. can i put a 12-27 shimano ultegra cog in the rear without having to get a new chain or adjust the deraileurs?

plz help.. it's double points this weekend.. hehe

EDIT: forgot to mention it's a 9speed ultegra group i'm riding on.