How does radioactive decay affect material properties?

The title question cannot be answered generally, unless the naturl decay chain to the final stable particles is given , and the time.

Within the question the example of uranium 235 at its half life can be answered by looking at the natural decay chain :

It is seen that it ends up in the stable lead 207, having lost through decays 38 nucleons. As alpha turns into a gas the material will be lighter by the ratio 38/235 . Radon is a noble gas, and it decays very fast into polonium, a metal. I do not think there will be time to create noticeable holes in the lattice by the radon leaving, as it decays very fast. Possibly the shape of the potential of the lattice may be affected. The stable end nucleus is lead which is a metal and will also occupy lattice locations. Maybe a specialist will answer with more details.

Other nuclei will behave differently, depending on their natural decay chain.