electrical ??

It's pretty clear that adding a 100A panel to a 60A service feed will not magically give you an extra 40A capacity, but the original question relates to potential fire hazard and there may be a circumstance that could cause such a hazard. As stated, the utilities will not normally provide a 60A service these days as 100A or greater is the norm and the utilities own the cable up to the mast in most circumstances. Suppose an existing 60A service was replaced by the electrical company. Since the modern norm is 100A or higher, the protection at the pole and the gauge of wire from the utility is 100A spec'd. This would not be a problem feeding a 60A service as the load expected is lower than the rating of the service feed. Now suppose the homeowner upgrades to a 100A panel in the house. What is left of the original 60A installation is the meter socket and mast wire. Now you have a service feed capable of delivering 100A, a house panel with a 100A main breaker, and mast/meter wiring capable of supporting 60A. Since all the protection is 100A, you could overload the mast/meter wiring, possibly creating a fire hazard. This is why I was so adamant about taking the proper permits for this sort of work.