We understand that this world is not full of absolutes and that changes to approved designs will be necessary, especially if you're employing split-testing...which you should! In that case, simply create a new task for the edit. Because baseline art was created, the edit should be quick and easy for you to open up a completely new task in no time.
If you have another task open and quickly need edits to an old task, work either has to be approved on the open task or it will be halted to complete the old task edit you need.
Regardless, no concurrent tasks will exist. We only devote our focus to one thing at a time. Here's a great article on MONOTASKING HERE and a TedTalk about it below: