While a labor cert is for a hypothetical "future" position and doesn't preclude you from getting promoted, your employer and you are claiming that after the GC is approved, you will be employed in that "future hypothetical" position - an Engineering position in your case.
Keep in mind that you can't amend the I-140.
If you want to be employed as a manager after the GC is approved, you should definitely amend the Labor Certification application. If Labor has been certified, then you need to file a fresh application since a certified Labor cert can't be amended.
My 2c.
Disclaimer: I am not a lawyer. You definitely need legal advice.