I was on E1 visa and am waiting for approval to change it to B2.
I want to go out of the country and come back later.
Will I be able to do it?
"Status" is something you only have while in the US, and determines your ability to stay in the US on that trip. Once you leave the US, you have no status anymore, and there is no "status" to change or "extend" anymore. (Any pending application becomes irrelevant and are automatically abandoned.) Even if you have already changed or extended your status, it doesn't matter anymore as you no longer have status after you leave the US.
Regardless of what status you may have had in previous visits, you will always need a valid visa to enter the US (with some exceptions like automatic revalidation after short trips to Canada or Mexico, or for Canadian citizens or people entering on VWP who don't need a visa), and the visa needs to be of the classification you want to enter into. So if you want to enter into B2 status, you will need a valid B2 or B1/B2 visa.
I talked to the USCIS. I will need to request a new visa from the embassy in that case.
More interesting, even if I travel out of the country after the change of status is approved, I would still need to request a new visa!