Yeah, sorry buddy, new trains don't make a new cred.
Think of any old woodie that's received Millenium Flyers in the last few years. They have new trains and have likely received far more extensive trackwork than Mantis ever did. They're still the same cred though.
It’s not really comparable to a woodie that received new trains where you’re still sat in the same position. Mantis you ride the coaster stood up, with a floor. Rougarou you ride sitting without a floor. Two different riding experiences. But then I suppose you also have those old togo coasters that had both a stand up and a sit down train.
Consider it a new cred if you like, the beauty of counting creds is that you can decide yourself if you’ll count it or not.