Be cause U atoms are heavy and migrate under the influence of gravity to the center. Their relative abundance steadily rising.

I don't know if is it enough to have some effect. But if we discover some exoplanets, hotter than expected, it would be indicative for a process like that.

I don't know, it just might be, that the radioactivity of a planet rises for a longer time than usually postulated. Certainly, the Galaxy's atoms are more and more radioactive.