Water change? What's that? ;-)
Well, we don't do them often, but when we do - here's the deal.
Sump is in the basement, so that makes life a lot easier.
We start by flipping the switch on the RO/DI output, so it fills a large rubbermaid container (25g). When it's about 1/2 full, we add salt, PH, and heater, let it mix, heat, and fill the rest of the way (a day or so).
Then, we drain (via tubing) water from the sump to fill 5 x 5g buckets. Dump them down the basement sump hole - the sump pump then pumps it onto our back lawn (hubby's endless effort to kill as much grass as possible). We then put a return pump in the rubbermaid, pump the new water into the sump, and turn the return pump back on.
Not as difficult as it sounds - especially since we do it once every 2-3 months (if we're good).