This may be wildly off, but I'm going to guess anyways:
A powerful wizard can turn an object into another object, that
1. has the same genus
2. can somehow be contained within it
Here's how these conditions would fit the given samples:
- Turn a plate into a pencil
Both have zero holes, and a pencil can fit on a plate
- Turn a ceramic bowl into a cue ball
Both have zero holes, and a cue ball would fit inside a ceramic bowl
- Turn a kettlebell into a rubber band
Both have one hole, and a kettlebell is often wrapped in rubber (except for the handle) to prevent damage to the floor; the wrapping rubber is essentially a wide rubber band
- Turn a plastic bag into a concrete block
Both have two holes (a plastic bag has handles, a concrete block has usually two holes, see picture below) and a concrete block would probably fit inside a plastic bag)
So unless I am completely in the dark (absolutely not ruling that possibility out), a scissor can be turned into
a wedding band (one hole each, a scissor contains a metal ring that you put a finger through)
and something that cannot be transformed into anything else could be
a Klein bottle, whose inside isn't distinguishable from its outside, and therefore it cannot contain anything. In addition to this, it has a different genus from all objects in the real universe.
Wish me luck! :-)