What possibilities with approved I-140

[happybuddha]

Hello, I posted this in the PERM section too, but then I think its more appropriate here ?
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Hello,

I have a few questions about the GC process.

Lets assume that a persons Labor is approved and say I-140 is also approved. The H1 petition is still valid till Oct 2014. Assume Priority Date to be Jan 2013 and category, the most cursed one, EB3.
Now, what happens if the person leaves the job in say April 2013 and joins another employer on H1 ?
What happens if the person returns back to home country ?
If the person is in home country when the PD becomes current, then what happens ?

Would all the answers remain the same if the person is still waiting on adjudication of I-140 ?

Cheers,