Here are some options:

Use REVDESIGN command. It will trace out an existing pipe run with a design line and you can re-fill with the proper pipe.

Trace the existing run of pipe with a 3D Polyline and use the LFILL command to fill in the proper pipe.

Use a script and rename the existing pipe to the proper name.  Use EXECUTESCRIPT command to run the attached script file.

You need to change the file extension to .cod to run.  The forum does not allow .cod files to be attached.