I'm sure UA runs with spare aircraft at some hubs. So, these aircraft are not in for mod, but rather being spares or regular maintenance unless there's a unspecified repair that must be made.
FYI:
On the UA Fleet Site, I post all new unit info and most wide and narrow bodied layovers, except at a UA hub unless it appears to be a mod operation. (Examples: SFO: only post 737NG/Airbus which look like a B-Check and 77Ws that look like PE mods - MCO all layovers are posted as they could become extended - IAH just show GUM conversions - Don't track LAX/EWR/IAD/HNL/ORD except for Induction)
https://sites.google.com/site/united...rvice-aircraft
To reduce the number and length of posts on this thread, compared to above, I don't post narrowbodies not involved in mod or paint. I limit widebodies posts to contract maintenance locations, mod and paint.
Sometimes I miss something and sincerely thank those who post on this thread to correct or add more information. This includes layovers worthy of notice like 764 N69059 in SFO for the past 20 days - I've posted N69059 on the Fleet Site.