NJ, have you considered the 'sheet method' which has been advocated here? (Place a sheet of plastic over the entire pool, pump the 'unwanted water out from under the sheet while simultainiously putting the fresh fill water on top - it allows you to keep the water level up without the dilution that Richard addressed). As Tenax wondered, how high is the cya and at what level do you want it?
Richard, thanks again for your knowledge and willingness to share it! (I actually didn't have to reach for my old chem textbook, nor the bottle of asprin , to understand this one !)