If it were me, I would change it. It's cheap and easy to replace.
My good friend is a big whig at Mitsubishi Motors U.S.A., the Director of Quality Control, and he has told me that the normal life of an O2 sensor is about 60K miles anyway, so by replacing it at regular intervals, you are just preventing it breaking down when you least expect it and have "driveability" problems.
2001 E430, Bourdeaux Red, Oyster interior.
1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".