Hi Carol,
Sorry to hear about the situation you are in but try not to get too stressed and take it one step at a time. You have been given some great advice here, theres so many caring people here!
My situation is kinda like yours but opposite regarding the order of the surgeries. I had scoliosis surgery first from L4 to T5. I was only 15 then (48 now) and I can honestly tell you the surgery was not to bad. And it was done very different then,alot of progress has been made in the procedures and results with scoliosis surgery are much better. Back then I was in a plaster cast from my hips to my chin for 9 months.....no more of that nowadays! But like I siad,it wasint to bad.
My second surgery was in 2008,posterior fusion w/hardware from S1 toL4. All levels fused good except the L3 L4 level and thats was only becouse of the prior fusion working as a lever. You wont have this issue

I had revision done this past May which was anterior/posterior with hardware to secure L4 L3 agian.....Now its the wait and see thing

Anyway thats my history about multi level fusions, now thats out of the way! lol
I think you have a very good surgion from what you have siad and since your possible new fusion will be above the old one it should be better but I would still discuss this with your Dr. and another thing I would ask is if he would have to remove hardware from your prior fusion to "incorporate" the two together. Sometimes old hardware can be in the way. My Dr siad he hoped he wouldint have to do this but the old hardware was in the way and needed to be removed. I hope I am not being to blunt but its a question that neeRAB to be asked as it does effect recovery time.
As far as recovery, I would assume that you would benefit from a rehab post surgery,I was in the hosp. for 6 days and rehab for 7 days...it was very benificial to get the rehab before I went home.
Well thats my story, ask your Doc those questions. Get all the info you can.