You can travel as long as you can use your H1B visa to return to the USA while EB2 to EB3 downgrade i485 is pending. It does not matter whether you apply for i131 advance parole or not. Some people use advance parole to enter the US because they want to avoid US embassy visa interviews due to the fear of H1B visa denials or delays with form 221g issuance. Note that we do not recommend using AP to enter the USA if you have filed i-485 with EB2 to EB3 downgrade as you will narrow your options to jump to the EB2 queue if it starts to move faster in the future.