Not yet… 737s retired largely as a result of downsizing / route cutbacks, and replacement by RJ (CR7/Emb170) flying. UA have an RFI out for 150 new narrow bodies, but no decision as yet – and in any case, IIRC that RFI is largely around a 757 replacement

Andy

Which would either call for the A321 or the B737-900ER.