travel when Priority date is about to be current

Here is the situation:
Currently on H1B (expires in another 3 years), but priority date is about to become current. Would like to travel outside of country around the time the priority becomes current, is there any conflict ? Does one have to be in the US when the green card is approved ? If it is approved when the person is out of country, what document should he/she uses when re-enter the country (H1B or green card ) ? Many many thanks