What's the difference between DIP25 and DIP1000?

[Emmanuelle via Digitalmars-d-learn]

Hi, I've been reading about DIP25 and DIP1000 and I'm not quite 
sure if I understand the difference between the two—is DIP1000 
supposed to be a rework of DIP25? And what's the difference 
between `return ref` and `return scope`? Also, will there be any 
compiler version where `-preview=dip25` and/or `-preview=dip1000` 
are the default?

Thanks!