My I-140 got approved on Mar 25, 2010, priority date Dec 20, 2007.
Current H1-B extension (this is my 8th year on H1-B) valid until Dec 21,2010.
Now my current employer is closing my location in June.
I have received a job offer from Canada.

Question:
Assuming I go to Canada for a few years and then come back to the US on a fresh H1 or L1 visa.
I know I will have to file for a new PERM.
But can I still use the approved I-140 from employer A to retain the old priority date ?
How long can I use the old I-140 approval to retain priority date ?