First you need to get an accurate CYA. The looking at this thread, you say it's "halfway to the scale that starts at 100." The test strip is pretty unreliable so I discount that. Done it small batches the water shouldn't mess up the lawn. I've done partial drains without problem. I too had a high CYA, right around 100. I am letting evaporation and splashy kids lower mine steadily and slowly. I just chlorinate up to 8PPM at night and let it work down to 5PPM and then move it back up to 8.