Box explains the man in Corinth changed within a few months, this is the basis for the change towards disfellowshipping.
The “Box” may “explain” the WT teaching but like the song says “it ain’t necessarily so”.
Apart from what is included in the Bible canon, Paul wrote other letters to the church in Corinth, including one between the first and second to the Corinthians. There is no concrete evidence that the man referred to in 1 Corinthians is also the one referred to in 2 Corinthians.