That's exactly what we did. We made it into the ensuite for the master bedroom as the family bathroom was bigger than that one looks.
But we did it without losing a bedroom as we converted the attic at the same time. So we lost a pokey bedroom that made a fab ensuite and gained a massive new loft bedroom.
You'd just be gaining a bathroom.
It might be better, if you are desperate for a bath to take some space out of the back bedroom or put a bath in the loft room.
In short, pretty easy to do, but you'd have to think about resale or whether you are going to live there forever.
Fab house though.