Borderlands 2 DLCs, is possible to replay them again and again?

So about Borderlands 2 DLCs, I know there are extra missions and enemies and they accord to your level when you entering the area first time. But I have a friend who entered Oasis zone from DLC at level 8 I think, and his mission from this zone accorded to level 15, now all the area is level 15 and all mobs too, my friend now is level 21 and the area and missions remain in level 15, he says now the area is useless until he play it in 2nd playthrough, my question is, now he can't change Oasis zone level according to his no? and is possible to replay missions on DLC zones in same playthrough? and to not have the thing happened to my friend, I should never teleport to DLC zones until I became high level to get good loot?