This article is from the FAQ, by with numerous contributions by others.
If the basis of granting the NIW is the work done while being employed
by a specific employer, then the alien worker must intend/continue to
work for the employer at the time of the GC interview (in general).
It may be possible in some cases to change employers provided the worker's
job duties and responsibilities will be similar to those while employed
with the previous employer and therefore, it is still in the national
interest. In all other NIW cases, changing jobs before the GC interview
has no consequence at all.