If a gadget breaks, gets lost or is stolen, it makes sense to replace it. But deciding whether to abandon an older, still-working device for a newer, shinier one can be a soul-sucking dilemma. Succumb to the temptation to upgrade too early and too often, and you can find yourself getting a piece of technology that is only marginally different from the old version, as well as a hefty credit card bill and the feeling that you played right into the hands of a corporation’s marketing department. So what’s the best way to decide when to upgrade?