Disable Required validation attribute under certain circumstances

I was wondering if it is possible to disable the Required validation attribute in certain controller actions. I am wondering this because on one of my edit forms I do not require the user to enter values for fields that they have already specified previously. However I then implement logic that when they enter a value it uses some special logic to update the model, such as hashing a value etc.

Any sugestions on how to get around this problem?

EDIT:
И да, здесь есть проблема с проверкой клиента, поскольку она не позволит им отправить форму без ввода значения.