trigger Contact on Contact (before insert, before update)
{
    for (Contact contact : trigger.new)
    {
        if (contact.Z__c == true && contact.X__c != null)
        {
            contact.Y__c = contact.X__c;
            contact.X__c = null;
        }
        if (contact.Z__c == false && contact.Y__c != null)
        {
            contact.X__c = contact.Y__c;
            contact.Y__c = null;
        }
    }
}