If you have an approved I-140 and your I-485 has been filed and pending for more than 180 days (or if you filed I-140 and I-485 concurrently and both have been pending for more than 180 days), you may switch to a same or similar job under the AC21 portability rule. Also, starting 17 January 2017, employer withdrawal of an I-140 pending for more than 180 days will no longer cancel the petition.

So if you can get as far as the I-140/I-485 filing, you should be good to go, though IANAL (and you should retain your own to guide you through this process).