The YouTube Partner Program has had it's critics since it's initial launch (I'm one of them), and the program itself has seen various changes as it's benefits became available to more users. This week has marked a dramatic change on the definition of who is a Partner.
The YouTube Creators Blog lists the changes to the program. The simple answer is that any YouTube user who has at least one monetized video is considered a Partner. With many users already having the ability to monetize at least one upload, this opens the door for a much wider group of Partners.
As for the other Partner features beyond monetization (such as custom thumbnails), these will not be immediately available to the 'new' Partners. These will have to be earned by the 'new' Partners in the future. These 'new' Partners will not have to complete the Partner application the previous Partners had to complete. The 'new' Partners are also eligible to apply for the current YouTube Next promotions. On the unfortunate side of things, many of these 'new' Partners have received a misleading email leading them to believe that they already have these features available to them. As of now, this is not the case.
For YouTube Partners who were Partners before the change, there is no change worth noting.