Lets take the example from the EPR paradox article, which mentions a positron and an electron occupying quantum states and becoming entangled. There are two observers, Alice and Bob.
In state I, the electron has spin pointing upward along the z-axis (+z) and the positron has spin pointing downward along the z-axis (-z).
In state II they are opposite.
Alice now measures the spin along the z-axis. She can obtain one of two possible outcomes: +z or -z. Suppose she gets +z. According to the Copenhagen interpretation of quantum mechanics, the quantum state of the system collapses into state I.
Meaning if Bob measures the spin, he will get -z.
My question is, what happens if Bob or Alice measure the spin along the z-axis again, will it remain z+ for Alice and z- for Bob or can it change between measurements?