A Clear Method should allow us to execute _changedValues.Clear();

    public T Clear<T>() where T : BaseProxyClass
    {
        _changedValues.Clear();
        return (T)this;
    }

Reason being following request:

var leads = (from l in crmContext.CreateQuery<Lead>()

                  select new Lead

                  {

                       LastName = l.LastName,

                       LeadSourceType = l.LeadSourceType

                  }.Clear<Lead>());

The only way to select column here is by creating a new instance of Lead in select. This will cause all columns to be considered "changed". Even though they were just retrieved.
Therefore it should be possible to Clear the _changedValues property.