He doesn't have to "turn" on Trump - he just needs to tell the truth - under OATH. Does Bolton honor the U.S. Constitution more than Trump? We shall see...
"Ambassador Bolton, what exactly did you mean calling Giuliani's doings in Ukraine, at Trump's direction, a 'drug deal'?"
That's going to be interesting.
Also, there is now so much testimony available, with more to come before Bolton appears, including from direct eye-witnesses, Bolton knows there's no point in taking part in a cover-up that is already failing, as lying will accomplish nothing other than to expose him. The Constitution has next to nothing to do with it. The only real obstacle is whether or not Bolton hopes for any role in any future Republican administration. In that light, he's probably very busy prepping for that testimony right now.