Being born in a smartha brahmana family, I have had the chance to observe and also perform many vedic rituals at home and other places, starting with nitya karmas like sandhyavandanam to major homams/yajnas like Ganapathi, Navagraha, Sudharasana, Mrtyunjaya homa. As I grew familiar with the rituals over the years, I have begun to notice that the procedure for the same rituals have become modified here and there, depending on which Vaadhyaar (brihaspati/purohit) is guiding the execution. Even sandhyavandanam seems to have been modified with some mantras which were never part of it since my grandfather's days.
So my question is who decides on changes to rituals, for what purpose and by what authority? Under what circumstances can they be justified?