Getting back to the printer,
http://www.archive.org/ has the user manual
for the 'TRS-80 Line Printer' which I am pretty sure is a Centronics 703.
This manual does cover replacing the ribbon, and yes, you do replace just
the ribbon, not the housing. Apparently, the new ribbon came packaged to
drop straight in, you then removed the packaging once it was in place.
-tony