I'm currently heavily using the Mongo_Db class for a huge WebApp project. I plan to follow the releases (currently, we're using the current 1.5.1) and upgrade whenever a new one is released. Will this class change in the future releases and will therefore need us to heavily change our codes?
General rule is that we don't change the API and guarantee backward compatibility for minor (1.x) releases.
For major releases we can introduce a new API (2.0 will introduce a new one for most of the classes), but in that case we will provide where possible an optional compatibility layer, so you can upgrade the framework and upgrade your application over time.
A good example of this is Upload in 1.6, which is swapped for the new 2.0 package, plus a compability layer so nothing changes for existing applications.