The biggest shift from PB OS 2.x.x to BB10 is probably more UI than anything; that magical land of "Cascades". It includes what is purported to be a more intuitive, easier-to-navigate UI structure itself, and a collection of native UI widgets that are supposed to make it easier for developers to create rich, intuitive applications. I qualify that a bit, but what I've seen already IS promising.
While it represents a version change (i.e. a major step-change) from the current PB OS, it will remain backward-compatible; most everything that runs on Playbooks currently should either be fine as-is or only require minor adjustment. Such apps will obviously not take advantage of the zoomy new UI stuff, and developers may want to update, but otherwise there should be no problem.
However, apps written for BB10 likely WON'T be compatible with the current PB OS...