Changing employer while extension is pending

[simivalley]

Hi,
Need some expert advise on my situation.

I am currently working for company X and my current visa is getting expired in Feb 2012. My extension will be filled by X by end of Dec 2012.
In the mean time I might be getting a new job which will start from early next year with company Y. They are going to start with visa transfer process sometime in Jan 2012, when my extension would be already filled by company X.

Is it allowed to change the employer when extension is filled and pending? Has anyone went thru the similar situation?

Any pointers on this will be very appreciated.

Thanks,
SimiValley