Before x

A: Can I go to the party please?

B: No, it is too late.

A: Please? I promise I'll be back before 10pm.

B: Fine.

A: Yes!

B: Now go before I change my mind.

For the last part of the dialogue, will the person actually change their mind even after the person goes? Or is it just a figure of speech to say that if the person doesn't go soon, the person will change their mind and not let them go.