You can change employers any time if you are a self-applicant and will continue to work in your stated area of national interest. But NIW priority date will take the same time as a normal EB-2 application does. See:
We would highly recommend not applying for an H visa if it can be avoided. In the past few weeks, I have provided consultations to various employers on H-1 visa refusals. The refusals were ridiculous, illegal and would be overturned if there were an adequate mechanism for reversal. So, I see no point in applying for an H-1 visa. Even if you enter on AP and continue working for the original H-1 employer, you ARE considered to still be on H-1. All other matters are more or less ancillary.