We have a production Rails & Angular app that is based on Bootstrap 2.3.2. It is an internal app right now, but we will soon open it up to external clients. It is stable and there is no look-and-feel reason to upgrade to Bootstrap 3.
To upgrade to Bootstrap 3 will take some effort, and it is more likely that when opening the app up to clients, we will engage a designer to work from the Bootstrap base and extend it.
As Bootstrap is "just" CSS and supporting js, I am wondering if there is any reason to not continue working with Bootstrap 2.3.2 for this app?
UPDATE I should add that the application is a keyboard-intensive, desktop (only) application used in offices. At this time it seems unlikely it will be used on mobile devices.
DECISION
We eventually decided to upgrade to Bootstrap 3, along with AngularUI. Bootstrap 2 was already looking dated... The upgrade was a lot of work, as there was no automated upgrade path. There are some conversion websites but we found they did not work well for complex pages and every page had to be manually recoded.