Getting PNP nomination does not change your current status. If you are currently a worker or a student then this status remains the same. So the restrictions on your work permit (for example if it is a closed WP) does not allow you to change an employer. Only an open work permit will allow you to change employers/work for any employer.
If you are a PNP nominee under any employer-driven stream, and becomes eligible for PNP partly by virtue of a job offer extended by a prospective employer, the method of getting you to be employed/join/work for the employer is by issuance of a work support letter along with the LoA. This support letter is used in lieu of an LMIA to get a WP, while the PR application is still under process. If you use the work support letter issued by the PNP this still results in a closed WP to the employer who gave the job offer. So you are still restricted to that employer who "sponsored" your nomination (by issuing the job offer letter), and cannot change employers.
Should you change employers mid-stream, then the possibility exists that a re-examination of the eligibility criteria for which your nomination was previoulsy attained (i.e. employee-driven stream, job offer from employer) would see that one of the basis of the nomination (job offer from from former employer) does not apply anymore, so the nomination and the eligibility may be/can be rescinded.
.../hth