Everything is negotiable in this lifestyle. You are here to enjoy it and not to ruin your lifestyle experience.
Depends on the nature of 'negotiation', I think. Breaking trust is not negotiation, and if you've got a hard limit you're trusting your top to not push it.
On the other hand... I had a sub many years ago and each year we reviewed our agreement. Various topics and activities ranged from 'yes please!' to 'if you want' to 'I would rather not' to 'you would have to make me' to 'please don't make me' to 'hard no'. We mostly stuck to the first couple, ranging into the other groups when she needed tuning up (sometimes 'requested' by being a brat). Or if I felt like it, to remind her she gave me that control.
In these reviews sometimes things got moved. In a couple cases things we tried things, either that weren't already on the list or had a tentative rating (i.e. hadn't actually done but she thought she could predict how she would feel about it) and adjusted the rating. Normally it was an annual review, but the first couple times were subject to adjustment more or less immediately. She always had the option of safewording out but never used it. On a couple of occasions we finished and she asked for a change.
"I thought I would be okay with this but I'm not."
"Okay, we didn't know."
Then she asked me to set up a scene from her 'hard limit' list so she could demonstrate her commitment to her submission to me. It was an activity that didn't particularly interest me and it was on her hard limit list, but since it was a direct request I set it up. I asked her about it and she told me that it wasn't submission if she only did things she liked, and even things she was just okay with wasn't really submission. For this demonstration she needed something more... she asked to suspend her safewords and that I only intervene if it looked like she would be harmed.
Afterward, she asked to make an adjustment... to add a 'please make me do this' rating.
"You liked that?"
"No! It was horrible."
"Why do you want me to make you do it?"
"Because it's horrible. It was humiliating and painful and degrading... and I came harder than I ever have, because I was doing it to prove I'm your bitch."
"So you want to do it again?"
"No. I don't ever want to do it again..."
"You want me to
make you do it. You're telling me that when I decide it's time, you'll be doing this again, you'll be asking how you came to find yourself in this situation, how I can stand being with someone doing something as filthy as that."
"Yes."
"Do you want to do that again, now?"
"... no..."
"You want me to make you do it now."
"YES!"
So... yes, I feel negotiation is part of the relationship, but it's not always the correct time. However, there should be room for situations where unknowns come up, either something not on the list because it wasn't thought of, or something where the rules are based on a guess that could be wrong. And it pays to be specific and explicit. When I first talked with her about limits she mentioned she'd rather not do something (that we're not supposed to talk about here) and I asked her to clarify: "is this something you will do if I tell you, but you don't want to? Or something you entirely want on the list of things that don't happen? If you want this to be a hard limit you need to say so, otherwise you're expressing a preference and I get to pick."
TL;DR a relationship like depends on and will not survive without trust. This works in both directions, but mostly from the bottom trusting the top, because the bottom is more vulnerable.