United is not interested in the A 318 at this moment, according to some of its employees have said to me. And if they are, they would have ordered it to replace its B 732s.
The B 727s will be gone in about 3 years time. But not the B 737-200s! Currently, there is no replacement for it. NOT AT ALL! They have just went through heavy maintenance and will be around for at least 5 years.
It is possible that UAL will order the B 737-600 or the A 318 in the future. As a replacement of the B 737-200s. If not, they may simply get more A 319/320s, and assign their B 733s/735s on routes that are currently flown by B 737-200s.
Hope this helps.