If you're looking for the change from one item to another item sharing the same item slot, see Subtype change.

Transform is a mechanic by which an item or skill turns into another item or skill without losing enchantments and without losing its "used" identity in relation to Confiscate Evidence or its relation to Slavemaster's Knout/ICE-Bot/Activated Improbaglitchity Device/Gnome/Slingshot/Fair Trade (Class Talent)/The Space Stone/Sabotage. Also, items created by The Reality Stone or Race: Voidcaller will retain their "fade away" property after being transformed.

Transforming is confirmed to take place due to the following:

If Unstable Mutagen is used on a Horcrux, the transformed item retains the Horcrux enchantment, and Weakened Soul is not given. However, using Unstable Mutagen on Marieke Doll will trigger the passive to give out Wrath Of The Admin.

An item chosen (but not yet sent) with Raven does not retain its "chosen" status when transformed due to Unstable Mutagen. Unlike Raven, listed items in the intro paragraph above retain their relation. Evolution Stone relation needs testing.

If an item is transformed while visible in another inventory due to Sabotage, it can still be chosen successfully. The Smoking Mirror/Three-Eyed Raven relation needs testing.