Your talking about the keel bolts? If they are J bolts imbedded in the keel pretty much you can't. Mars Keels has a system of melting the old bolts out and replacing them, but you have to ship the keel to them and that's probably worth more than the boat. Generally what's done is you drill holes for new bolts next to the old ones (called "sistering"), then drill holes in from the sides so you can put nuts and washers on the new bolts inside the lead keel. It's also a major big job and maybe worth more than the boat. I lucked out on my 1969 Columbia 36, they used a harder than usual lead alloy and hangar bolts tapped into the lead. It's the only boat I know of that was done that way. I was able to replace the badly corroded bolts without much difficulty.
This is the only real advantage of iron keels, you just unscrew the old bolts and replace them.
