Quick question, does changing out O2 sensors make much of a difference? Also what are the negative consequences if they are not working right or disconnected?

The guy I bought the 2002 Forester from said that he had had a mechanic replace the front O2 sensor but I found it was not connected when the Cat converter clogged, could that have caused it?

Most of what the seller said was untrue.  So far have replace most all the exhaust system but had the muffler pipe welded instead.  New radiator, some radiator hoses.  Plugs, wires and coil.  The major mechanical seems ok.

Front O2 sensor does the fuel trim.  

Rear checks how well the system is working and has no impact on the trim.  That's my understanding anyway.

If the engine was running way rich, it can cause CAT problems.

the computer primarily uses feedback from the MAP/MAF sensor and the front O2 sensor to adjust fuel:air so, yeah, a bad or disconnected one of these could easily lead to a failure of a cat conv or possibly even a fire. Older cars often have a lot of grease and oil underneath and cats can get cherry red under bad conditions.

yeah, front one is important, rear one not so much