How can I swap wires/pins on generic aftermarket O2 sensors?

I would go to a scrap yard and find those connectors on a scrap vehicle and make up a "crossing" adapter to fit between these O2 sensors and the vehicle connector.

The cheaper ones, apparently have a short life and you may want to purchase the OEM ones in the future - because the clips and bits go brittle over time...

Yes, it is possible to swap the pins on the female connectors.

You must be very careful not to break the little plastic tooth that holds the metal pin in the connector.

First, carefully pry out the white socket cover from the inside of the connector with a flathead screwdriver.

This will expose the four pins and the tiny plastic tooth that latches into a hole in the flat side of each pin to hold it firmly in its socket.

Use a flat jeweler's screwdriver to push the sensor pins (black and grey) slightly down so they disengage from the hidden tooth on top, then out through the back of the connector.

Be very careful not to apply too much pressure until you are sure the pin has disengaged. If you break the plastic tooth, the socket will no longer hold a pin firmly.

Repeat with the other pin, noting which pin was in which socket.

Reinsert the pins in opposite positions, pushing firmly so they lock back into the connector. You should be able to tug on the wire from the back of the connector firmly without the pin coming out. Snap the white socket cover back in with the original orientation.