I'm a Canadian working in the US on the TN status. I already have a priority date from the I-140 approval notice, the I-797. However, I plan to leave my job, go back to Canada and come back to the US for 6 months as a visitor. By the end of the 6 months period, I plan to get another job in the same field as my previous job in the US on the TN status. Does this affect my priority date or invalidate my green card application? I understand I can transfer my priority date if I simply switch from one company to another while maintaining my TN status within the 2 month grace period, but I'm not sure what happens if I let my TN status elapse.
I'm not a lawyer and none of this is legal advice.
Your current plan sounds like a very bad idea if you want to get your Green Card quickly. But before we dissect that lets cover the main question that you have about the retention of your priority date.
If your I-140 has been approved for more than 180 days then your employer can no longer revoke their application and you can retain this priority date.
In a hypothetical situation, your new employer would first need to be willing to file an Employment Based immigration petition, once the Perm was approved by USCIS, they would then be able to retain that priority date.
Now, lets back to why your plan is an extremely bad idea. The TN visa does not allow for dual intent, once you've filed for an Employment Based immigration petition Travel in and out of the USA become tricky from a legal standpoint. (I know people who have done so without issue but that is not to say there isn't inherent risk)
The core issue is that after you've established immigration intent, you are no longer eligible for TN status or to really visit the USA as a visitor.
Here's a hypothetical interview with a CBP official at the border -
CBP - Sir, what is the purpose of your visit to the United States
You - I want to visit for 6 months being jobless
CBP - Hmm, how do you plan to support yourself?
You - I have plenty of savings, have worked in the US in the past etc
CBP - Interesting, you've worked in the US in the past but have no plans to work on this trip
You - No, not at all
CBP - I see that you also have an approved I-140 immigrant petition. Do you intend to immigrate to the US
You - Yeah, I want to immigrate. Just not on this trip. After 6 months I will find a job
CBP - So basically you've been living in the US for the past X years and plan to continue to live in the USA
You - Yeah, but no.
CBP - I'm sorry sir, you don't satisfy the Criteria of being a visitor to the USA and I'm denying you entry. Next time please come with your complete immigration paperwork.
My personal recommendation for you is to continue working with your current employer, have them file the I485 when your date is current. Unless you were born in India or China this should happen relatively soon. Once you have your green card you can choose to be jobless for however long you want.