Let me try to explain what I mean money-wise,
For 3-day hoppers, you paid $237. For $255, only $18 more, you can upgrade them to 7-day hoppers.
To upgrade them to 10-day non-expiring hoppers will be $416, meaning you'll shell out $179 more than you already did. The 10-day non-exp. hopper will cost you $161 more than the 7-day hopper that will expire.
So you'd be paying $161 extra to have 3 days leftover for the future. For your next trip, this would be a savings of $76 over buying new 3-day hoppers. The question is, when is your next trip? If it's going to be a long time, think about if you want to give Disney an interest-free loan during that span of time. But if it's only going to be a few months, then I think it's a good deal.
Me? I'd stick to spending only $18 more to upgrade the tickets for my current length-of-stay, because I'm not into loaning money to Disney. But I know there are those who disagree with me.