Unfortunately this is how it works in Durpal. Sinatures won't get appended to existing comments automatically. I'll look into modules and extensions for drupal later, maybe there is something which would enable such a feature - although i doubt it. 

Cheers!
Paul. 

Yes, this CAN be annoyong... "The spread of evil is the symptom of a vacuum. whenever evil wins, it is only by default: by the moral failure of those who evade the fact that there can be no compromise on basic principles."