Interestingly, Arrested Development did actually switch Ann actresses, but not in order to replace Mae Whitman.
Perhaps the switcheroo is a sign that the Arrested Development team fully intended to change actresses each time Ann popped up until they found Mae Whitman and didn’t want to let her go.
If Arrested Development ever manages to move past potential complications and come back for that long-awaited Season 5, maybe Ann could appear once more.
While it might have made for a fun running gag to have a slightly different Ann whenever the character made an appearance, it’s difficult to imagine anybody other than Mae Whitman in the role.