Weekend mail call:Alex from Los Gatos, Calif., wants to know if I think the San Francisco 49ers could consider trading 2015 free agent Mike Iupati this offseason.Bill Williamson: I can see why you ask. Re-signing Iupati may be difficult with high-dollar contracts for Colin Kaepernick, Aldon Smith and Michael Crabtree coming up. I would not be shocked at all if Iupati leaves in free agency in 2015. He is valuable piece, but you simply can't keep everybody.