I'm thinking that grading tumors is a little subjective and these change with time. Perhaps the first reading of low was on the cusp of something higher and between then and the most recent reading it transitioned higher. From what I understood, it was whether or not the tumor had penetrated the muscle that was most important.
I wish your husband all the best going forward but losing the bladder is not the end of the world as I can testify. Mayo will do the best for him and if it comes to it; I would encourage he consider a neobladder procedure. Mayo is on the leading edge.