I’m still on 3.0 so this is so useful for me, thank you!
Since I haven’t done it I can’t say for sure, but SimpleApplication is dead and for now we have LegacyApplication, but the way forward is to extend application. See here: jMonkeyEngine 3.1 alpha 4 released
I also had to re-create some of my .j3md files when I briefly tested a 3.1 alpha many months ago since some stuff has been removed but netbeans or java or w/e got all bitchy about it.