Exactly at which point does that valence electron in the silver atom get its spin, in the SG experiment? When it enters the magnetic field or when it hits the detector/screen or when it's actually observed, by someone!? How can a magnetic dipole align itself antiparallel to a magnetic field, in the SG experiment and the MRI machine, and then stay like that, thru any number of such SG magnets!? Unlike a compass needle. It can't be diamagnetism or a gyro/scopic effect.
I'm afraid we cannot answer exactly when the electrons flip spin, due to the Uncertainty principle. What we can establish is an experiment which tells at that moment what is the spin. If you send spins through a Stern-Gerlach apparatus that separates into $+$ or $-$ components and then send only one of the components $+$ through an identical Stern-Gerlach apparatus you would see again only $+$ components, if you disregard external interactions.