The 100% water change would do more damage than good. It would upset your whole balance, cause a new cycle in the main tank and most likely would not cure any disease or problem that may be associated with your fish, in fact it may just kill more fish during the ammonia part of a new cycle alone, not to mention stress them making them even more ill.
Honestly the QT is the correct long term answer, that and finding out just why your fish are dying.
What symtems did the fish have? ick, fungus..etc..?
Thomas