How to convert a ubyte to a byte without losing information?

[CharmingChocolate via Digitalmars-d-learn]

I can cast a ubyte to a byte, but as far as I know I'd only get 
positive values out of that conversion. If I was instead getting 
a ubyte out of a function and I want to assign those bits to a 
byte and have some of those numbers be interpreted as negative 
values by the new type they're in, how would I cast that in D?