Disable Required validation attribute under certain circumstances

This problem can be easily solved by using view models. View models are classes that are specifically tailored to the needs of a given view. So for example in your case you could have the following view models:

public UpdateViewView
{
    [Required]
    public string Id { get; set; }

    ... some other properties
}

public class InsertViewModel
{
    public string Id { get; set; }

    ... some other properties
}

which will be used in their corresponding controller actions:

[HttpPost]
public ActionResult Update(UpdateViewView model)
{
    ...
}

[HttpPost]
public ActionResult Insert(InsertViewModel model)
{
    ...
}