To be frank, how much or "if" a premium will decrease depends greatly on the insurance company and the associated fire risks in your neighborhood. So to get the most accurate information, you need to contact your own agent for information. The agent will be able to review your files and will help you make a decision regarding the best type of roofing for your home.
If you're located in a forest or highly wooded area (for example, parts of Los Gatos and Saratoga and Monte Sereno), then replacing the roof with something less flammable could certainly yield some cost savings. However, if you're in a densely packed suburb, as are most homes, then the chance of fire beginning from a source outside of the home is probably less likely than from a source WITHIN the home. These risks will already have been factored into your home insurance premium, so the costs may not decrease greatly or at all.
So check with your agent for the best information. Also, I might add, that my experience with buyers is that they're not as enamored of a metal roof on a home as they are of tile, composition shingle or wood roofing. If you want to factor in "buyer appeal" for roofing, you might want to also consult your trusted Realtor for assistance.