Old experience letters for I-140 with new employer

[immi_help_needed]

Hello,

I have an approved I-140 with Company A and am in 7th year of H1B.  If I join employer B:

1. Can I use the experience letters I had used with the previous employer's i-140 or do I need to get new letters from all old employers? What all documents are required for application?

2. Is it a risky process to get a new i-140? Are there more chances of denial?

3. I did Master's from the US and I have not traveled outside US since I started working on H1B and now I am in 7th year.  Can I go to Canada to get stamping or do I have to go back to India as it will be 1st time stamping?

Your help is appreciated.

Thanks.