How much trouble would it be on my 93 impreza L (sedan) to change the rear drums to discs?

there are a couple good write-ups in the subaru conversion forum on http://forums.nasioc.com

 

may want to check that out, as well as this page

http://wac.addr.com/auto/obs/disc/disc.html

How much trouble would it be on my 93 impreza L (sedan) to change the rear drums to discs?

There's no question that disc brakes are better than drum brakes. BUT 80% of braking is done by the front wheels! Don't expect a big improvement for your time and money.