I do realize that 31.8 is stronger and I would go there if necessary. But I prefer not to spend the money to replace my s-works carbon bar & Thomson stem with new stuff unless it's really necessary. But if the consensus is that people replace carbon bars every X years as a rule, then I would go ahead and get a new 31.8 stem at the same time and be even more sure.

I think that the handlebar is the single worst component to fail on a bike. Your entire upper body is relying on the bars being there. If they fail, you're going down and hard. Unless you get lucky and your wheel turns and throws you sideways into who knows what, you'll go down right onto your stem and broken bars. That would not be good.