What makes a theory change over a period of time

A theory is simply something that is approved by a large amount of people with research. A theory theoretically can never become a law because in order to become a law, It would have to be proven without a doubt. Meaning that in order to change over time, the only thing that could possibly change is the amount of evidence and people that support it. For example, Thermodynamic Laws are proven and can be researched without a doubt. However, Darwin's Theory of Evolution will likely never be a law because it can not be hunkered down and completely proven.