Well, if you don't take control of your life, she will. Until she leaves. Take a sick day off, or a vacation day, if it can be done so soon.... and advance your career. Call in sick...take a vacation day...it doesn't matter at this time, as you want to get ahead.
Consider, though, how soon after the meeting you want to attend is she leaving? Do you have the fortitute to stand up to her during that time? What is her relationship with the incoming boss? Will this influence her attitude towards you and will you be able to manage your anger?
Or, you could always ask her what is going on with her. Then you would know and perhaps not be so angry. But,hide your cards and intentions of what you want to do...unless it impacts your future there, there are always excellent ways for negotiation. Now is the time to practice them. Call me if you need help, or email me.
Thank you,