Hi, R4nd0m, yes, this has happened in the past. Also, existing thralls were reduced (remember the Bandit Leader?).
It appears to be the complexity of changing existing vs changing new that affects which way the devs went. It matters little, because attrition will eventually take over. Wipes, deaths, etc, will slowly kill off the existing ones and all will be in balance, for at least 5 minutes 
You can compare old to new by following this process on this page.
Regards
jim