Urgent_Can I substitute Xanax to discontinue Ativan?

I was prescribed Ativan to help with Insomnia.I thought it was a sleeping pill.I have since found out that is not the case.I have been taking 2 mg of Ativan for 5 months.It does not help my insomnia at all.I want to stop taking it but discovered it is dangerous to abruptly stop.Incredibly, the doctor who originally prescribed it is now retired.He was not in a group but had an individual practice.I have enough Ativan to last for 4 days and obviously have a critical issue.My original plan was just to stop taking it and now I cannot get an appointment with a new physician for 21 days. I have some Xanax that was prescribed for panic attacks. I only take the Xanax for panic attacks and typical only take 1 mg or ½ mg of Xanax as needed which is usually only once or twice a month.Sometimes I can go for several months without panic.When I am out of my 8 Ativan, can I take th Xanax until I can see a doctor in 21 days without having withdrawal problems from the Ativan? If so, what dose of Xanax should I take? I am so worried I will have a life threatening situation when I run out of the Ativan. Thank you to anyone who can give me advice on this.