While i have no doubt it is good to replace coolant, the root cause seems an overheating event to me. Overheating resulting in a warped block and coolant able to get deeper into the gasket.Yes... replacing the HG as a PM, before it blows.
Given the theory is a slow degradation of the HG coating due to "old" coolant, this would have to begin at a coolant passageway (water jacket) eating it's way through HG material until it breaches into a cylinder. There is no way to really tell this deterioration is occurring until a breach occurs. At which point, obviously, your HG has blown.