Many, many posts have been written about C27 keel bolts. In answer to your specific question, the lag bolts are to be added in addition to the existing keel bolts, not to replace them. My personal opinion on this subject is that most of such repairs are probably unnecessary. Over 6000 C27s were built, and I have never heard of a documented case of a keel falling off. This includes all those that have had absolutely zero maintenance or care in the last thirty years or so.It certainly can't hurt to add the lag bolts, and definitely would add security, but is probably not really necessary, no matter how rusty they look.