When/why to upgrade?

I have a pair of Dunlavy SC-IV/A speakers that, as far as I can tell, work the same as when I bought them new in 2000. I would upgrade if I had confidence that replacement speakers would work better for me. But that would require considerable effort and expense, and is not something I would undertake just for the novelty effect. Over the years, I've upgraded amps and DACs, and always with positive results, but speakers seem different. How do you folks know when it's time to upgrade yours? Dunlavys, after all, have a great reputation for producing neutral sound, and I don't see compelling evidence that speaker design has advanced much, at least, within my price range.