The rule of "irrevocable I-140" is attached to AC 21 rules where the employer loses the ability to revoke the approved I-140 or approval I-140 once the adjustment of status (AOS) aka I-485 form is filed. However, the USCIS reserves the right to still revoke the I-140 approved petition based on "fraud and misrepresentation".
Therefore, make sure that in case you need an I-140 to file for extension or transfer beyond six years, your employer has not 'secretly' revoked such a petition.
We recommend people before they make a move to actually consult with an attorney experienced in I-140 and H1B issues. Our law firms has handled hundreds of those, please call us at (510) 742 5887.