My work permit is locked to my employer, my stream is employer driven one. If you got PNP through your employer, your new work permit will be locked as well. I am not sure if "locked" is a good world

but it describes it very well, i can only work for my employer and if I'd change it, my PNP nomination will be revoked, thus PR application would be F***ed up.
With work visa support letter you can get your new work permit if you need one. I do not think that it has an expiration date.
I received my PNP nomination a few months before my PGWP expiration date. So I applied for new work permit right away, I got "locked" work permit for another two years.