I'm not for demoting Bello to AAA or even the pen. To me, he's the 2nd best starter in the rotation, or tied with Houck for that slot. (Sale is #1.)
Kluber has been doing better since his first start and much better his last 3 starts.
The 5 slot is Paxton vs Pivetta, and to me, Paxton need to prove he's better, which should not be too hard, right now.
If Paxton gets shelled, I'm not convinced Pivetta to the pen is the right move.
I'm also not for making big decisions based one one Paxton start, but I do think he deserves a shot. Ideally, he should start in the pen and earn a starting slot, but he's not a RP'er, so I wouldn't mess with that.
It's not an easy call, but I'd give Paxton a few starts. Maybe go with 6 starters for 1 or 2 times through, or just put Pivetta in the pen, now.
I do worry we might be messing up our most durable and consistent pitcher in 2 years, by moving him to relief, but I see not better choice.