“In theory, there’s no difference between theory and practice. In practice, there is.” -Yogi Berra
Scientific theories are a dime a dozen, yet only the most successful have a large range of validity, hold up to many disparate lines of evidence, and stand the test of time. Inevitably, our explorations go beyond that range of validity, and a new theory must come along to supersede them.
But does that invalidate the old theory? And does that mean it would be irresponsible to bet your life that the idea you’re working on is correct? While no theory can ever be called “fact” beyond a doubt, there are good reasons to believe that some theories, like the Big Bang and evolution, will never be overturned. Still, it’s always possible, and that’s part of why we do the science in the first place.