Since some people have reported drops in Cyanuric Acid levels even when there was Free Chlorine (FC) present, I thought I would track more carefully my CYA level this summer along with CH and other parameters that could act as a proxy for water dilution. My fill water has a CH of around 50 ppm and I have very little evaporation since the pool cover is on the pool most of the time. I've monitored my splash-out (dilution) level from the CH and it was minimal at far less than 10% over the entire summer.
And yet, my Cyanuric Acid level fell from 25 down to somewhere guessing at around 10 ppm. I noticed that the drop "seemed" faster around the time I needed to do some shocking because the water looked a little dull (I had let the chlorine drop too low, forgetting to add chlorine often enough) and was also faster at the lower CYA levels because I was still maintaining the same FC level so the FC/CYA ratio was higher than usual.
I had earlier reported on The Pool Forum (here) about Patent 4,075,094 which is in this link that indicates that Sodium Hypochlorite can break down Cyanuric Acid (CYA) in a molar ratio of 4.5:1 (which is 2.6:1 by weight and where optimal reaction conditions have a molar ratio of 6:1 to 8:1) and at a pH of 9-12 (with optimal reaction conditions at a pH of 9-10) with reaction time of hours. Shock levels of chlorine do not approach these ratios or pH, but nevertheless it is still possible for the reaction to occur slowly over a period of weeks and months. I also speculated in the post on short-term reaction exposure from manual chlorine addition, but the exposure time is quite short (minutes).
So I would guess that there is a slow reduction in CYA over time, probably due to the oxidation of CYA by chlorine, and that higher chlorine-to-CYA levels, such as found during shocking, may somewhat speed this process up. The drop I saw was higher than I expected, but it seems quite real and without an alternative explanation.
While I wouldn't say it's a great way to reduce CYA, perhaps super-shocking a pool (to at least an FC of 60% of the CYA level -- similar to yellow/mustard algae shocking) and raising it's pH could work to reduce high CYA levels, but would probably need to be kept that way for days, perhaps a week. It would just take a LOT of chlorine. Doesn't sound practical though drain/refill isn't exactly simple or cheap either.