If you change a data source in an app from multi to single cardinality, what happens to for example update filtered selection action nodes? Will they work with an empty filter?
If I create a new Update Object node with the single cardinality data source, it will not have the option for a filtered selection.
Good question! In fact, that filter will be ignored when changing from Many to One, since when doing Update Object on a single-cardinality data source, there is not option on “which object to update”.
But, as you see, the preview of the action node still holds the “filtered selection” and the filter is still there. In other words, we don not currently clear that filter, and you will still see it, but we are considering whether to block/warn changing cardinality when in use, or to leave it as it is. E.g. if you by accident change the cardinality of a data source that is heavily in use, it would be very unfortunate if we cleared all filters towards that data source 
Changing cardinality and other settings that other parts of the model are using as a basis of their configuration can, in general, lead to unpredictable behavior. Sometimes it may work fine, but the reality is that you then end up with other parts of the model (other action nodes, etc.) that have an invalid configuration because the initial conditions that it was configured from have changed.
That is why we consider locking such properties from being changed. Now you need to be very careful about these types of changes.