Quick update. I got busy and didn't get to this till I was forced to when the garage started getting smelly every time the car pulled in. At that point, diagnostic codes were p1165, p1259 and P0138. Got a good deal on converter and downstream sensor and had them both replaced by a local shop along with an oil change, which the car was due for. Its been 3 weeks since and no CEL. Fingers crossed.
Here is my guess as to what may have happened.
1. First the CEL was for upstream sensor
2. It took me a few (~week to 10 days) days before I could swap it out.
3. That must have impacted the converter due to excess gas burning at the converter
But can't explain why it kept complaining about the upstream sensor even after replacing it.