UAL has already retired the ones it doesn't want/need and replaced (cough) the domestic ones with 737-900ERs except for the PS (EWR-LAX/SFO) configured birds. FWIW, FedEx bought a bunch of them for cargo conversions.

There really isn't a direct replacement aircraft being manufactured (yet) to replace the RR powered frames for the markets they are being used (mostly trans-Atlantic).

The oldest remaining one is from '89, the newest '00.

https://sites.google.com/site/united...fleet-tracking