Hi Scott;

Given that you have extremely high CYA and 7 year old water, I'd definitely go with water replacment. 'Used' pool water does build up residue which tends to make it harder to work with -- things like phosphates, nitrates, and who knows what else.

It's this residue that is behind some of the push to measure TDS, despite the fact that TDS meters (really, conductivity meters) can't distinguish bad 'residue' from benign ionic content, like salt.

Replace as much as you can at one time -- you won't get nearly the effect from replacing 25% 2x as you will from replacing 50% one time.

But, you've got to be safe and draining your pool can definitely destroy it, depending on type and situation.

Ben