new exhaust/O2 sensor

i'm putting a high flow cat and a flowmaster on tomorrow. i've seen where many people suggest replacing the O2 sensor at the same time. situation:

first, i have a universal catco cat i got from a freind..don't know much about them, but it has a small, plugged up, fitting sticking out of the side of the cat. is this for a new sensor? if it is, my stock cat has the O2 fitting on the back of the cat (aft). will this make a difference in the way my computer reads this input? or do i need to get a bung welded in the same place as on the stock converter?

second, i've been under there with pb blaster for the past few days in preparation. it appears that the O2 sensoris attatched with a nut, but i can't tell for sure (everything is pretty corroded down there). anyway, i tried to loosen it a bit today...not gonna happenwithout a blow torch...and then there goes the wiring. is this a situation were i should just go get a new O2 sensor. and if so, can i just cut this off at the wires and splice a new on on there?

its a '97, sensors on the downtube and the rear of the cat. no in-between.

btt.

Well i cant answer your question about before or after but to heat it up use a small propane torch and put it on the bung itself or just start it up and let it run for about 20 min

it was running hot when i tried to loosen it. does anyone know if this thing is even removable? is it welded? i can't tell, it is too corroded.