I mean reasons other than being bored of the set up.
What are the tell tale signs that is is time for the tank to be tore down and substrate changed? Are there any good threads for ideas and tips on revamping a tank?
Leak? I'm suspecting that would be the right time. Otherwise... snails! Those little pond snails are a real pain in the butt. If your substrate provides buffering then once you notice it not working would be a good reason, however, with root tabs and a healthy tank I don't believe an inert substrate is a bad thing.
I agree, even substrates that have lost all their buffering capacity (such as old ADA soil) can still be used. From personal experience, most planted tanks can go for years without having to be torn down. The only substrate condition that comes to mind which may cause issues is anaerobic bacteria.