A few more datapoints and observations.

Just opened my United app and discovered that all 3 upcoming UA itineraries I've booked have been affected by schedule changes, all by 20 minutes or less.

1) A bit surprised I haven't received any e-mail notifications for any of these changes.

2) I called in to see if I could change to more convenient flights, and despite the schedule changes being under the official 30-minute policy, the agent was happy to oblige (all old and new segments in this case were on UA). I was pleasantly surprised.