help: my priority date (pd)

Hi,
I recently changed my job from one area(financing service) to bioinfomatics. My previous green card application had to stop (at the step of ready- of 140). My friend said I can use my last priority date (pd) when I do the new application.

I want to confirm that here.
And I also am wondering how to know my pd? Could I leave everything to my new lawyer? I really don't want to talk to my old company to get that info.

thanks.