It wouldn't be frustrating for me and I like people who are willing to experiment with their bodies and limits and interests. "It's only pain, and it will subside"
I think if someone can be articulate about the concept of experimenting and the possibility that one may get in over their head, it would be a lot more fun than finding out the hard way. (safe word, they ask for a lot more and hit the wall really fast) Playing with the idea of "let's see how far you can go" is a great theme, particularly between two people who have good chemistry and can be a very exciting roller coaster to ride.
I feel like all of us should be willing to experiment. And the truth of the matter is every day is a little different physiologically - one session a person might be able to take a lot more than the next. It all depends both on the person dishing it out and the person taking it.
I never have trouble switching up, it's part of the process of the time I spend with someone. Sometimes things just don't work as you expect them to, sometimes people have bigger fantasies than they can take in reality, other times it's fun to push something as far as you can take it because the relief in the submissive's eyes and body when doing something else is so enjoyable to see.