I agree with the attorney. You may have to start a new

I140/I485 application with the new sponsor. All that you need to do is exactly what you have done for your first I140/I485 package, except that you might not be able to submit your I485 immediately.

Since you application strongly ties you up to a sponsor, I also suggest you to try some other categories, like EB1A or EB2-NIW, which will allow you to change job much easier while keeping the submitted/approved I140 with you.