The generation name didn't change, only the poses. The item simply evolved how it was supposed to. You will get the poses back once the item finishes its cycle. See if this explanation of how evolving items work helps you:
Say for example there is an item with four generations.
The first one to be released is the first gen, which (for the sake of simplicity) has one pose, a scarf.
When the first gen evolves, it loses the scarf pose, and gains a shoes pose. At the same time, the second generation is released, which will then have the scarf.
The item evolves again, the first gen loses the shoes pose and gets a hat pose. The second gen gets the shoes pose, and a new third gen is released, which has the scarf.
The item will then evolve again, and the first gen will finish it's evolution and gain a jumper pose, but it will also unlock all the old poses. Meanwhile the second gen now has the hat.
The later generations (second, third, fourth and so on) will continue to evolve until such time as they finish their evolution cycle and gain all the poses. They will simply be a step behind each other.
The only real different ones to speak of are the Rapid evolving items. With those the first gen is released and it will evolve every few days for about two weeks or so (normally it'll evolve on a Tuesday and Friday). The poses will be added on with REI's though, and when the first gen is finished evolving, the second gen is released already fully evolved.