CYA levels drop only by dilution (evaporation doesn't count!) or by bio-degradation (pool slime).

But, you can't simply let your pool get slimy for a couple of weeks, because the end point of the bio process can be nitrogen gas, which is no problem, nitrates, which aren't good but aren't an immediate problem, OR ammonia, which is a HUGE problem. We don't know why sometimes it goes one way, and sometimes another. MOSTLY, it does NOT go to ammonia. But, on a occasion, it does. We had a lot of pools with zero CYA and high ammonia early this pool season, and they are a nightmare to clean up.

So . . . the safest route is a partial draining. You'd need to drain about 1/2 the volume of your pool, to go from CYA=150 to CYA=80 However you must be VERY careful with a liner pool. Your liner is held in place by the water, and if you drain it, you can lose it. When the liner is first installed, it's very elastic, but over time, it loses it's stretchiness.

With a liner pool, you can usually safely drain it to 8" above the HIGHEST horizontal liner covered surface. On most pools, this is the floor of the shallow end. But, if you happen to have liner covered steps, you can NOT drain the pool at all, without professional help.

Your other option is an in-place draining, which is safe, but a time-consuming, fidgety process.