So, how would YOU like this to happen?

I'm hoping they are just alternate evolved evolutions with different movepool and slightly better/mixed stats.

I think in-battle only would be kind of silly. Would you have a limited use? If not why wouldn't you just do this every battle? Which is why I hope they are permanent. Absol has been my favorite and most epic Pokemon partner this I started playing Pokemon, he needs an evolution not a in-battle only form.