If I am using both of these upgrades, can they both trigger in the same round? Reveal a green, bb8 triggers, then use stay on target to change it to something else making it red?
BB-8 and Stay on Target
Yes. BB-8 only requires you to reveal, not execute. What happens to it after you reveal is inconsequential.
Yes, the combo works like that.
However, you should realize that you can't trigger BB-8 when you're stressed. So you can't BB-8 and Stay On Target two rounds in a row, unless you have some other way to get rid of stress (like a wingman with Wingman).
Ah, thanks to you both. I hadn't really thought about how useless both cards would be with a stress token though. Hmmm, I'll have to cook up something else to use with stay on target I think.
What about Ello Atsy and stay on target? Are the T Rolls still white if I change to one?,o
What about Ello Atsy and stay on target? Are the T Rolls still white if I change to one?,o
No, they become red.
What about Ello Atsy and stay on target? Are the T Rolls still white if I change to one?,o
No, they become red.
Just adding the ruling 
Q: If two or more game effects that change the difficulty of a maneuver conflict, which effect takes priority?
A: An effect that increases the difficulty of a maneuver takes priority over an effect that decreases the difficulty. For example, if a ship equipped with R2 Astromech is dealt the Damaged Engine Damage card, all of the ship’s turn maneuvers are treated as red maneuvers, including the 1-speed and 2-speed turn maneuvers.
Just adding the ruling
One thing I'd like to point out, because people have made this mistake before.
The above ruling only applies to maneuver difficulty. It is not a general rule that applies to everything. That may seem obvious to some, but it's a mistake people have made in the past.
They misremembered the rule as 'any negative effect trumps a positive one'