2.5L ('98-'01) Exhaust manifold for '98 2.5L?

[1999Mazda Central Florida]

I'm getting set to replace my catalytic converter on the '99 2.5L. It sounds like it either has a hole or it's cracked, and I have slight loss of power. Looks straight-forward (2 bolts at the exhaust manifold) and a clamp with springs on other end.
The question is: would it be a good idea to just replace the O2 sensor (since I don't think it has ever been replaced.) Are o2 sensors from Rock Auto direct replacements?
Also, would messing with O2 sensor effect the computer? I'll probably just disconnect the battery before doing anything (like all manuals say to do.)
Also, these hangers look hard to get off...just cut them and replace them with new ones?

 

[Fredness]

Crack in the cat/flange leak causes air to get sucked in, between pulses, and causes the O2 sensor to get bad readings - it can cause lots of problems.
I would change the O2, if it has been more than 75K since you did it last.
Prefer Motorcraft - if you can find them.