I partially agree with Khodges (1/2 agree
). Drop 1" in diameter, but keep the same pitch (agree with Khodges) -- or, keep the same diameter & drop the pitch 2".
In your example of a 12-8 2B, go to either an 11-8 3B, or a 12-6 3B. That is assuming that the prop is made by the same manufacturer, of the same material, & has the same general blade shape & aspect ratio -- otherwise, all bets are off.