If you use your ticket at least once, they will credit you full gate value of whatever ticket it is (the 4/3 deal does not require an actual 7 day ticket, and then you can add hopper or no expiration or both or neither). If you do not use the ticket at least once, you are credited whatever you paid for the ticket, or in cases where not purchased directly from Disney, whatever the ticket broker paid for the ticket.
The best way to upgrade to APs under the 4/3 deal is to purchase a 7 day ticket and add no expiration. You get the free days, which isn't much, but then you get a decent discount on the no expiration option since it goes up for every day on the ticket and you're getting 3 of those days for free. With the extra days not being much, but the no expiration going up considerably, I wouldn't go over 7 days in that case. Use it once as I state before, and you'll get full gate value. You'll wind up with ~$75 discount on your AP. If I remember the math right.
We do it all the time. DH gets a sizable discount on tickets through his HR department so we purchase there and upgrade to APs every other trip. As far as it being worthwhile, that will depend on each and every person. For us, we go once a calendar year and I make sure two trips are on each AP--as they are good for one year from activation or first use of an upgraded ticket. An AP is the least expensive ticket media when you go more than 11 days in that year, assuming hoppers. Each of our trips are 10 days.