See clip from Attorney Rajiv S. Khanna's conference call video that addresses this question.
In this scenario we have employment based people who are good workers, very skilled people and they have a lot of trouble changing jobs or advancing in their careers because technically while the green card is going on even if you take up a promotion you could end up losing your current green card and have to start all over again - at least part of the process. So this is not a good thing.
Keeping in view with the current law what Obama had said on November 21st about his plan and finally what this bill (Immigration Innovation Act) says.
First of all under the current law if you change your employers before your I-140 approval is obtained you will lose everything. You have to start all over again. That means if you are a EB-1 based candidate your I-140 must be approved. If you are an EB-2 or EB-3 based upon the labor certification your perm application must be approved and then you’re I-140 must be approved. Remember I-140 can be filed by premium processing. So you got to get your I-140 approved very quickly. Generally speaking if you move before I-140 is approved you get nothing. If you move after I-140 is approved but before I- 485 is filed you keep your priority date and you keep the right to extend H-1 beyond six years as long as the I-140 is not revoked. This is an important concept to remember. You keep your priority date - that means when you go to the next employer you will have to start the green card all over again but your priority date will be the date the first case was filed which again is a very big advantage. The priority date is capped but here is an additional provision. if your priority dates are current and you have filed I-485 with the old employer, after 180 days of the I-485 pendency and the I-140 approval you can take any job anywhere with any employer and be able to carry the same green card forward without having to redo any work. so before I-140 you get nothing, after I-140 and before I-485 you get your priority date and the right to extend your H-1 beyond 6 years and after I-140 approval plus I-485 filing 180 days thereafter you get the right to keep your green card even if you take a slight promotion with another employer or move to an entirely different employer as long as the jobs are same or similar.
President Obama said normally applicant can only file the last step - the I-485 if the priority date is current. He said he would change that by allowing the applicant to file I-485 along with the I-140 no matter where the priority dates are. This is a big advantage. That means technically just roughly speaking once you start your green card process through PERM in a year and half you could be free to join any employer – proximately or two years at the most as long as you maintain the job category you are in. President Obama also said he is going to make it clear that advancement in career is considered to be same or similar job so that your green card will not be disturbed. And that idea has been very welcomed. Combine that with the Immigration Innovation Act you could be getting your green card much earlier than 3, 4, 5....10 years that you are waiting for right now. So all these things combined are very good signs for people in a situation such as the query above. If you have not yet got your I-140 approved and you are far from that stage it’s ok as long as you don’t have H-1 problems. If you are about to get the I-140 approved then get it approved. If you have H-1 issues beyond six years make sure the second H-1 is approved before you change employers. Once the H-1 is approved for 3 more years even if the old employer revokes the I-140 they cannot take your right to the H-1 you already got. You can start another green card and protect yourself.