Then, if you change your job, your I-140 will be void unless the employer will hire you back. If you still work for th employer, but at the relocated work site, your employer needs to file a new PERM/I-140 all over again unless you will come back to your current work site some time later. Your employer also needs to do the H1 amendment in this case.
If you change your job, you can do the H1 transfer using the approved I-140. Unless your current employer withdraw the approved I-140, you still can use it to extend your H1 continually even if you work for another employer.
Talk to your lawyer if necessary.
