If you delete a party you won't lose any character progression, but you will lose party progression. While this isn't important in terms of acquiring Feats, it is significant for Heroic and Legendary progress and for the Legendary completion reward. Or that's my understanding and my experience of being forced to delete a party (actually, having a party deleted during an upgrade)
Yes this is true, I should have mentioned that. I suppose having to replay a few heroics isn't that bad since the other choice is not getting to play at all...
Also some people with this issue might not have any heroic or legendary progress at all.