The utility function is an abstraction that does not capture the richness of the behavior of agents in the real world, but the better an agent is at rationality, the more accurate (and comprehensive) the abstraction of the utility function becomes IMHO at describing the agent. I suspect that it never makes sense to model a utility function as changing over time.
Or maybe it makes sense only as a mental shortcut to be used only when we do not have time to make a proper analysis. People of course discover new wants and new preferences as they go through life, but this can be taken into account by saying (or noticing) that the person does not know what their (unchanging) utility function is in every detail, and every now and then he or she learn a previously-unknown detail (or fact) about their utility function.
The different agents at those different timepoints surely have different utility functions, don't they?
IMHO, no.