Thanks Ben. I've already completed one drain/refill cycle and am midway through the second. Drained off about 28", which I'm estimating is about 40% by volume (funny how that works). Drains pretty slow since the waste valve is a standard hose bib and there are no valves in the pump/filter/heater area that can be used to force more water to waste (would have gone faster before cleaning my filter when the back pressure was 35psi rather than 14, ).

I figure 4 passes like this and I should be in good shape. I did verify that my CYA dropped to around 150-175 after the first cycle.