Hope everything works out for you!
What is the category you are applying in? EB1,2,3...?
Priority date is only useful at the time of applying I-485 (the last step) and it becomes relevant only if the category in which you are applying is over-subscribed. Right now all the employment based categories are current thus priority date is irrelevant (I\'ve heard that for 2002 the categories will remain current, beyond that who knows!)
I-140 approval is valid for life. Thus, if you can wait at your old LC location until you get I-140 approved, you\'re home free!
Once again, talk to a competent lawyer!
Goodluck!
-Paresh