^Are you meaning a real WC loop, or a closed-loop LCS like the H70?
Closed-loop systems 'shouldn't' need to have coolant changed...it's why they are developed and sold as being 'closed'. Opening them up voids any warranty, although there are lots of people that have modded H70's with other waterblocks, reservoirs and bigger radiators. The pump is still pretty weak, and doesn't perform as well better suited watercooling solutions.
For $130, you get into the
XSPC Rasa 240 kit that will blow away the H70 in terms of performance.