Rice's affiliation with the number 80 is best identified with the 49'ers. IMO, when you leave the team you are historically identified with, your number, then, should be irrelevant. I know that's not how the players feel, but that's the way I see it.
Besides, the Seahawks uniforms are so butt ugly to begin with, why should he care? Whatever number he wears, he is going to look like a piece of sinus phlegm anyway.....