Yes, if you switch employers before the I-485 is filed and is pending for at least 6 months, you will lose the ability to adjust status on the basis of that approved I-140 even if the employer does not revoke it.  You'd have to restart the entire process from PERM if you switch employers although you might be able to retain the original priority date.  If you still have 2.5 years left in H-1B status it is still enough time to get the PERM started with the new employer and get it filed at least 1 year prior to the maxing out on H-1B, which is how you'd be eligible for extensions in H-1B beyond the 6 year maximum.