58bug,
Not only does the rear window have to be removed again but also the two rear side windows. The headliner material gets glued to these areas to help hold the headliner in place.
As far as how hard it is: That all depends on the person that is installing the headliner. If you have patients, GOOD glue, patients, heat gun, patients, good with your hands and patients then you might be able to replace it yourself. Otherwise this is a job for a professional installer.
Hope this helps you decide.
Enjoy life to the fullest - in a VINTAGE VW