Maybe something similar to this?
Manipulate[
If[dir != oldDir, a = -a; oldDir = dir];
a,
{{dir, 1, "direction"}, {1 -> "Positive", 2 -> "Negative"}},
Dynamic@Switch[dir,
1, Control@{{a, 0}, 0, 2, Appearance -> "Labeled"},
2, Control@{{a, 0}, -2, 0, Appearance -> "Labeled"}
],
{oldDir, 1, ControlType -> None},
TrackedSymbols :> {dir, a}, AppearanceElements -> None
]
