I would not use Play 1.x for Scala applications; its non-blocking support is complete crap (and I was pretty displeased to hear, when I asked in their IRC channel, that it would never be in 1.x, oh and by the way to use 2.x you'll need to rewrite your app).
You don't need to totally rewrite your app, but it's not backwards compatible either. The biggest change is that they've switched the default views to be written in Scala instead of Groovy. But if you'd still like to write the views in Groovy or are porting an app from 1.x, there are a couple of guys in the community who have written plugins to support writing your views in Groovy.
I can't speak to 2.x.