CYA Replace water? Or, just Add?

[Xebec]

The most excellent Pool Calculator says I need to replace 12% of my water to reduce my CYA from 57 to 50.

Is it sufficient to simply ADD water, or do I actually need to empty and replace?

Also, how much difficulty is CYA of 57 going to cause my vinyl AGP?

 

[JasonLion]

CYA at 57 is fine. It will go down slowly over time.