Hi Michael, this is similar to what we discussed in another post. When using QuantumState[arg1,arg2], if arg1 is already a quantum state object, the code means transform it into a new basis which is arg2
So if you want to transform plus and minus states into Pauli-Y basis, you should do this:
QuantumState[QuantumState["Plus"], "PauliY"]
which is not the same as QuantumState["Plus", "PauliY"]
