At the worst, you'll have to mill out a broken stud and/or retap for an m8 helicoil. Not necessarily both.
I've had to do that to several turbos. Removing the old studs / bolts is the worst part of any turbo refurb job for sure....
Good part is that once you replace all the old studs and nuts with stainless and brass/bronze, and use anti-seize, it's never difficult to remove again.