When switching between two base layers of different projections, OpenLayers will attempt to reproject the centre point of the map and adjust the zoom level to try and match the bounds of the current view as closely as possible. It also reprojects any vector layers that are added to the map.
When a required transformation function is not found, no transformation of coordinates will take place but the layer will not be prevented from changing.
No checks are made to ensure transformations are within the ranges of the projections specified.