“It is, however, an Ikkar Emunah that all Dinim remain the same L’olam Va’ed “
Not only is it not an ikkar emunah, it is a false statement. Do you use toilet paper? Did you recite the shema on your wedding night? When was the last time you heard of a bechor animal being identified for a mum? When do you think the texts of selichot or tachanun were standardized? Can you divorce your wife without her consent?
MOST dinim don’t change. But a few have.