That does refer to the body of Christ, whereas the other does not.
So you finally admit that the words "in Christ" means the Body of Christ.
And that is exactly what we read here:
"So we, being many, are one body in Christ, and every one members one of another" (Ro.12:5).
Let's look at the following verse again:
"Therefore, if anyone is in Christ, he is a new creation; old things have passed away; behold, all things have become new" (2 Cor.5:17).
So we can understand that Paul is saying that IF ANYONE IS IN CHRIST then he is in the Body of Christ.
So we can know that when Paul speaks about those "in Christ" before him he is saying that there were others in the Body of Christ before he was.
So are you now going to argue that even though others were described as being "in Christ" before Paul that they were not in the Body of Christ despite the fact that Paul said that IF ANYONE is in Christ he is in the Body of Christ?