Regarding chlorine oxidation of CYA, see Degradation of Cyanuric Acid. It take about 2.5 ppm FC to oxidize every 1 ppm of CYA. Lowering the CYA by 150 ppm would take 375 ppm FC so maintaining a high chlorine level over quite an extended period of time. It's not usually a very cost-effective way to lower it particularly if water replacement is not too expensive. In 12,000 gallons, 375 ppm FC is 35 gallons of 12.5% chlorinating liquid which at $4/gallon would be $140. 12,000 gallons is 16 ccf so water replacement would be comparable in price if it were $8.75 per ccf.