I think it depends on what the definition of "hard" is for a limit.

As an example. I had a horrid experience with tickling. One I will never repeat and so there for, tickling in any form, will ALWAYS be a HARD HARD HARD limit to me, and even with discussion, etc. the chances of that changing are less then 1%.

That being said though... there is a difference in "pushing." I am extremely ticklish in many assorted areas so a Dom who is "testing" to find the spots because he wants/needs to know where they are and the reaction, and immediately stops the moment he is able to make the determination is completely different then someone who knows and keeps "testing" for any other reason. As an example... if I shared that information with a Dom before a scene, then throughout the scene, he might test different areas lightly to see what the reaction would be and if they fell under the "ticklish" section or not. Such as slowly dragging a flogger across my body, noting when it touches my foot that I move, even a slight bit and the next pass with the flogger skipping the foot entirely would be a sign to me that the Dom is attempting to learn the exact limits, etc. and yet respect them in the process. So, yes, in a way the limit is pushed but only pushed to the point of completely understanding of it. If the Dom were to continue to drag the flogger across my foot, knowing I move from it, is when I would halt play and it would be done because he was not respecting that limit.

Now, there are other "hard" limits that I'm not as strong against. Do I wish to explore them, not really, they hold zero interest to me or scare me in some way. But, for the right Dom and based on very open communication, I would agree to the limit being pushed if ever so slightly, since I am also of the belief that limits, etc. change over time.

So... it's all in the definition of "hard" imho.