I have two bottles of milk in my fridge. One is nearly empty, about half a glass left. But recently expired. One is recently bought, and just opened.

Question is, what would happen if I pour the expired milk into the new bottle? Will it cause the new milk to go bad? Or will it not make a difference, since the new bottle is almost full, and has presumably quite a lot of preservatives that would make it sustain half a glass more of nearly non-preserved milk?