I140 approved and not revoked by X employer , I did change employer to Y from X and switch back to employer X

Hi,I am working with an X employee as a fulltime and did apply for H1B extension and it got denied on December 2017 , where as I have I40 approved and valid I94 with X employer. So did apply transfer with Y employer and it got approved before the I94 end date and got I797 approval notice with extended I94. So now I started working with Y employer as subcontractor to X employer for the same client. Its good till now and my X employer asking me to join back as Fulltime position to them in coming one month. I believe it will consider as transfer to them again. So if I join back to X employer as fulltime , do they need to start GC process again from PERM step or the old I140 approved one with X is gonna continue with them even i off on their employment for couple of months.

I appreciate your information on this.