The 732 aircraft are parked and the 733/735 fleet is far too young for replacement, with many years of service left. UA is having major financial problems, so I think a 733/735 replacement program is hardly a priority at this point in time.
UA has a large fleet of A319 and A320 aircraft, with more on order, and these aircraft have replaced the 72S and 732 on most routes. (Some routes of course have been dropped and others have gone to regional jets.)
When the time comes to select a replacement for the 733/735 arrives, the order could go many different ways: more A32X aircraft is one choice or the 737NG could be purchased since UA has a huge 737 fleet and flying the Airbus and Boeing narrow-bodies side by side would be no big deal. Both Boeing and Airbus will fight very, very hard for this potential order and much will do with price and delivery schedules. Anything could happen here, so never say never.
Another possibility would be for UA to look at the "large regional jets" for many current 735 routes, that would depend on its mainline operations and what the unions would agree to. Looking ten years ahead, its unclear whether mainline or regional carriers will operate aircraft with about 100 seats.