Hi Mandi :
I am going through the exact same thing as you are. I was on Celexa for years till it stopped working. I was on 60mg and I'm currently weening off of Celexa @ 10mg decreases per 7 days. I'm currently on 40mg of Celexa and going to be upping the Zoloft to 75mg (I started on 25mg).
I am finding that since this last decrease to 40mg of Celexa I am very anxious. I discussed this with my doctor and she informed me this was normal and in the meantime I was instructed to take clonazapam/Klonopin as needed.
I've had 2 different pdoc's over the years. My last pdoc would not ween me off an antidepressant onto another. I would just stop taking the current antidepressant and start another. The pdoc I have now is weening me off and onto another antidepressant at the same time. I don't really find much of a difference as both methods caused anxiety.
Try to hang in there and know what you are feeling is normal. You will level off in time. If you are having unbearable anxiety I would definately contact my doctor.
Take Care