I Agree, if you are going to copy the preferred address to the alternate the most likely reason is because you want to put in a new address.

I would like to see it either remove all data from the address when copied or prompt you "do you want add a new address or make the preferred address invalid"

If this suggestion for all new preferred data to be cleared doesn't make it to priority status, I'd like to refine it by starting with replacing the "date from" in the new preferred address with the current date just as the systems saves the current date for the "date to" in the now alternate address. Setting the "date to" to the current date was right on for the now alternate address so why not keep the same logic for the new address "date from"? Should be current date for both and because one must drill down under the "more" button to change the dates, the preferred address "date from" rarely gets updated to the current/correct date. This is an important piece of research data in order to assess the chronological moves and locations of a constituent.