Upgrade postgres without messing with ports?
Given step 9 in the link you provided:


  9.Run pg_upgrade ... Obviously, no one should be accessing the clusters during the upgrade. pg_upgrade  defaults to running servers
  on port 50432 (sic) to avoid unintended client connections.  You can
  use the same port numbers for both clusters because the old and new
  clusters will not be running at the same time. ...


It looks like it is just an available option.