Changing certain behaviors is different then pretzeling yourself into someone you are not.
Stating that 'no one should change into anything they are not for someone else' is different from what you state above. Sometimes we need to have faith that trusting someone else and attempting to change is the right thing. For example, an addict may not always want to go into rehab. They sometimes do it for their loved ones such as during an intervention and then eventually embrace the change when they see the positive changes. Also, sometimes pretzeling yourself into someone you are not is the only available option.
Yes, everyone has their list of non negotiable things that they won't compromise and that is completely fine. However, as mako mentioned it is a spectrum and there are things we compromise on a daily basis for others whether we like it or not.
'Who you are' is also influenced by circumstances and external factors. You could be a different person at 5pm on a Monday than at 5pm on the Friday of the same week You are changing constantly and so if the definition of 'who you are as a person'. The difference is often in your attitude towards the required change not the decision to change itself.