This depends on what you mean by "flexible with changes" and "resisting changes".
The PMBoK assumes a process in which you define the work, do a breakdown of that work, then build a plan to perform the work. It's a predictive model in which you define scope, cost, and time at the beginning of the project. So obviously, once you have decided on what you are going to do by laying out a plan, any changes to that plan can cause potential issues.
But the reality is that there will always be changes. Any project manager that assumes plans are written in stone and must be protected at all cost is delusional. It's not about resisting things, but keeping them managed and under control. That's why "change requests" exist (for whatever reasons). You must decide what to do with each change request. Some change requests will be accepted, some will be rejected, some can affect the time and cost in a negative way, some not so much or in a positive way, etc. Not all changes are a go, and not all changes get rejected.
It's not about resisting changes or protecting the plan, but about introducing changes in a controlled and coordinated manner (otherwise you get some things like scope creep which are uncontrolled changes in scope). In fact, as part of building your plan, the PMBoK includes the "Change management plan" that lays down the way you will handle the change control process and can define the roles and responsibilities of what's called a Change Control Board (a group of decision makers that review and approve/reject change requests - the PM may or may not be the part of this group).
If a change request is rejected, then it's just a matter of sticking to the plan you have. If a change request is approved, the project manager will then have to make the necessary changes to the plan to incorporate this new piece of work.