Alright, I’ll stay on topic. When I got the Valkyrja transmogrification, the items where it says 99 days next to it, were sent to my gift box: Valkyrja transmogrification pack, 500 crystals, 50.000 mana, and 50 energy.
After that, I tapped my monster list and went straight for Katarina. You know the way you set your rep? You tap on a *-shaped button next to the basic stats, and you see a list where it says: Set Rep Monster, Lock Monster, Transmogrify. By tapping on the last option, you go straight to a special page where you can preview the Before and After version of your mon, only if you already purchased the building that you need to mogrify them. It’s cheap anyways, I think I paid 3000 mana for that building.
When you have chosen your mon and have the necessary items (in my case I didn’t need them because I got the whole pack), you choose to mogrify your mon and the proces of transmogrification will look something similar to awakening your mon.