I don't want to quote too much of the Athletic since they are behind a paywall, but there are a number of quotes by Army in JR's article about the Faulk trade that give reason to believe the intention is not to replace Petro with Faulk:
Intended usage this year: "But while we are healthy, the ability to role out Dunn, Bouw and Gunny on the left and those three guys on the right, it’s going to be three good pairs that can play against anyone."
But more importantly, Army's quote about his conversation/negotiation about an extension prior to making the trade: "Credit Justin and ultimately his agents, who understood what we were trying to accomplish,” Armstrong said. “Going in, I was quite honest that we have two high-profile (unrestricted) free agents (Alex Pietrangelo and Brayden Schenn) that our goal is to try and sign and to do that, we needed him to be understanding of the overall cap situation and he was. We’ve seen some recent contracts that he was willing to come to our group at a cap hit we could afford with an idea of trying to keep our group together moving past this season."
The fact that keeping Petro came up during their negotiation AND the immediate plan to play him on the 3rd pairing tells me that both Army and Faulk are comfortable spending a ton of money on the right side of the D in order to create what they believe will be the NHL's best defense.
Trade for Justin Faulk gives Blues' defense 'most balanced...
As always, I highly recommend signing up for the Athletic. I have my qualms about JR, but the Athletic as a whole is well worth the subscription.