Erliest and safest time to change employer

[priya2]

Greetings!
I have recently started my green card processing via the employer sponsored perm application. The recruitment has not yet started, but is planned to begin soon. My question is when would be the earliest and safest stage to switch to another employer without affecting my green card? Can I switch once the labor cert is approved? Do I even have a chance to switch if its an employer sponsored perm application?

Please let me know.
Thanks in advance