There is a phenomenon that some of the posters (but not all) on this forum experience that is just what you described--varying levels of CYA at closing, with none on opening. Most of the time the ones that lose the CYA have algae, and the algae will break CYA down over a winter season. Other than an algae outbreak, I really can't explain why it happens, but it does. It doesn't happen in my pool, (but then again mine stays open year-round), but one of the other moderators, Watermom, has it happen in hers. Weird, huh?

If your test kit has been stored in a dark, cool place, the only thing I would worry about replacing is the dpd powder, and even then only it's clumped up or dark gray in color. The rest of the reagents should be fine.

Janet