[usernametbd]

> Green Card process has to be restarted from the square one

That's not fully true AFAIK. The green card (GC) is a 3-step process - PERM, I-140 and GC itself.

If you already have your I-140, unless your previous employer cancels your I-140 (which cost them a few thousand dollars), your new employer can just start over the third step of the process. This is not optimal, sure, but it is not square one either.

[woodrow]

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).

[suryacom]

That is not correct. The second employer has to do all three: PERM, I-140 and GC.