I played a game today against Skaven. he cast an inresitbe force for the 13th spell, on my temple guard and slann. he trows enough to change them all into skaven. Is it possible to change a slann into a skaven becaus he ins't a model on foot? But he is still infantry. We played it was possible. Probably he won the game with this but dind't feel right ( althou i killed a hellpit and a doomweel with a solardon) Thanks a lot!
You played it correctly. The spell mentions units of infantry, and the errata clarifies, saying all infantry models (including characters) are turned into rats. It mentions nowhere about only models on foot, just infantry models, so Slaan is kill.