Happy Wars

Evolving an item will turn an existing evolvable item into a new one. It also increases weight, overall stats, and sometimes adds an extra buff slot, but it will revert the evolved item back by 9 levels. If you previously used statues to get your item to level 15 you do not have to repeat this process a second time once the weapon has been evolved. To evolve an item, it must be evolvable and at least level 10 or higher. After meeting the requirements, the player would be given the option to evolve the item in the Item Level-Up menu, the cost of evolving is 5000 Happy Stars or 13 Happy Tickets. This cannot be reverted.

List of all available weapons that can evolve.

http://happywars.wikia.com/wiki/Item_Evolution

Need to Evolve a new weapon post update. This will sadly not pop retroactively.