I just read your posts. I too come from a family whose name was switched from Diaz to Dias. I am told that it was when my g (or) gg grandfather arrived in America, he was given the opportunity to "Americanize" his surname. So he did just that.
I am also part Portguese and part Spanish.I am told that one of my greats was from Portgual and the other from Spain...no wonder they fled to America.I believe that was an unheard of marriage.Both sides boasting of so much heritage pride.
I realize that this post is 5 yrs after yours, but perhaps someday you will return and we can discuss this.