Borderlands – Upgrading Borderlands 2 to Borderlands 2: Game of the Year edition for DLCs

Once you take a mission the level for that mission is set in stone. However all missions after that mission and missions you haven't taken yet will continue to scale as high as they're set to scale. See this answer for DLC level ranges in playthrough 1; most max around 30. In playthrough 2 the DLCs will max at 50, which is also your max level. Since you're already at the end of the main storyline you'll probably just want to start playthrough 2 before doing the DLCs.

The DLCs do reset so you can do it again. However, its beneficial on the second playthrough to complete ONLY the main storyline missions first, this brings you to a new mode called playthrough 2.5. In playthrough 2.5, all mission and rewards will be set to the max level of 50, so you'll gain much better unique weapons than if you completed them at lower levels.

You must not even accept a side mission until you reach playthrough 2.5 as the mission level is locked in as soon as you accept it.