wait. Before you go through the trouble, did you check out this page?
http://mike.focke.googlepages.com/antifreezeforyourboxster
is the zerex you got the extended life version? if so, I would think that's ok. basically, you want a coolant that's silicate and phosphate-free iirc.
otherwise, for flushing, I would just run a gallon or so of distilled water.
hopefully others will chime in about whether you even need to replace the zerex.