신뢰 등급 잠금/잠금 해제, 어떻게 작동하나요?

In the mean time I found my answer here: Unlock Trust Level downgrades user from TL2 to TL1

But if I lock him let’s say to 3, does this means he cannot ever reach 4 ? In other words locking works both ways or only locks so he is not downgraded ?