I think
Dirty Dog hit the nail on the head especially with the 'muscle memory is a real thing' and 'harder to make small changes' comments.
A lot of good points here though; I'm going to stay away from the 'right' and 'wrong' commentary because as we've seen these depend on the style, the person, the stance, the situation, etc. For that matter, I've seen techniques taught at white belt in kung fu that didn't show up until black belt for TKD. <Shrug> Different teaching and different styles.
That said, some folks are talking as if the human body and mind were computers and it just isn't that simple. That's why we model brains with neural nets. A computer will accept different parameters and do different things. A human mind takes those new parameters and says "is this right?", "Did I mis-hear?", "Maybe I should still do it the other way", and that's before taking into account the difficulty of the mind executing through the body.
Thought exercise - you are at a party, and you are getting a drink. You *need* to keep your drink separate from everyone else's. Do you A.) Take a red cup from the stack of red cups or B.) Take a glass with a completely unique design on it?
When you take a red cup, even marking your name on it, the brain has difficulty distinguishing between them, despite the minor difference, and you increase the rate of errors (picking up other people's cups to see if it is yours). The very different and unique glass stands out. This is just a thought exercise of course, but it can illustrate how the brain can have more difficulty with minor differences versus large differences.
Meanwhile on muscle memory another example then a physiological discussion. My wife practiced Shotokan for several years when she was a kid and before we met. When she joined me in TKD Chang Moo Kwan, some things were very easy to convert, others quite difficult. Several of the early kata are quite similar between the two styles. Since she would often find herself mid-form turning into a move from Shotokan kata, she had to work extra hard to execute properly (for the new style).
Now why is all of this? Some of it is the brain's desire to attach to patterns, and the more you've practiced the more ingrained it is. It is difficult but not impossible to turn the course of a stream that's only been there for a short time, but try to change the course of the Colorado river in the middle of the Grand Canyon.
The other part is that darn (but wonderful) muscle memory. The more we train movements the more the body builds the myelin sheath around the nerves on the way there. That's the layer that 'insulates' nerves (and connections in the brain too). The more built up that layer is, the more smooth and quick your movements are and the more your muscle fibers can be recruited in sync with each other to perform a movement. This is one reason that people who just start exercising can usually make surprising strength gains quickly - they aren't building muscle in those first 3 months or so, they've built a better myelin sheath.
However, that does mean that the actual nerve (and neural) pathways for those movements are more ingrained. You can cut a new path (learn completely new movements) without having to overcome this obstacle, and yes, new movements that are similar can benefit from the years of built up myelin
once the initial difficulty with modifying the movement has been overcome. You have to overcome both psychological and physical barriers to get that new movement in properly.
On the soft vs hard thing - ugh. Yeah, it's such a difference in the way of moving and thinking. I went from all hard style (which was not my first choice but just how life worked out) to bagua. Now, as I understand it baguazhang was often 'layered over' hard styles, but geez, it's tough to change the movement patterns and I'm still trying to get more flow rather than the darn rigid strikes and stances I grew up with. It probably didn't help that I was mixing in with Israeli combative work at the same time which is much more hard style, but still.
