Hi Retired Mom,
I'm sorry that I didn't reply before now. I understand your fears and confusion about why the new ortho might want to order the CT scan and myelogram. It's fairly standard practice to do this once a fusion has been done and to evaluate the fusion status.
I've had three of them so far and am on my way to my 4th, and I was scared to death, especially the first two times. So scared in fact, that I got off the gurney outside the myelogram room and tried to leave the hospital..... 
The reasons behind the ordering of the CT scan and myelogram are at least two fold, the myelogram shows any compression of the spinal nerve roots where they exit the foramen and to evaluate any compression of the spinal cord. It also allows them to see the fusion, far better than the CT scan. The CT scan is done right after the myelogram, and then you go back to your room to rest until they tell you to go home. The CT scan is done to check on the solidness of the fusion and to see if there is any compression of the spinal nerve roots from a different angle. The MRI can not evaluate whether or not there is a solid fusion so that is why he wanted to get it done.
The myelogram is scary, no one wants anyone to stick a needle into the spinal cord if they don't have to, but honestly, the hardest part was laying still. I barely felt it at all, and I think the only reason that I felt anything was because the lidocaine hadn't worked completely. They are usually done by a interventional radiologist, who does them all of the time. And the horror stories we read or hear about only make them all the more scary. But they truly do have a purpose and if the doctor really didn't feel it needed to be done, he wouldn't have ordered it.
Drink lots of coke , not caffeine free either, you want all of the caffeine you can tolerate. Tea, coffee, and soda all are terrific to drink post myelogram.
As someone mentioned, because you have had difficulty with the contrast used before, you would have to tell the person scheduling the myelogram about it. I know they used omnipaque during my myelograms, so I am not sure if they are still using that contrast, but they can give you medications, including benadryl before hand to manage the symptoms if you did have a reaction.
I know that having to face the possibility of another surgery if you aren't fused is a huge scary thought, I have had to deal with a failed fusion myself, so I have traveled that road too, but if it isn't fused, and there is a problem with the screws or the rods, it could endanger your spinal cord and that isn't a chance you want to take. I had two screws loosened up, both of them backing out of the vertebre and if they had come out any further, I would have had some more serious problems, and I've already got enough.
I was loosing what little ability that I had to walk and each day was getting worse than the one before, but having the revision for the failed fusion and replacing those screws and the rest of the mess that my 2nd surgeon had to clean up was the only choice that I had, or resign myself to permanently using a wheelchair 24/7.
You say that you can walk now, and you want to keep it that way. I hope that you will give yourself a little time to think about it and then decide to find out what is going on. Then you can decide what to do with the information. You don't have to decide everything all at once, and maybe you will be very lucky and find that nothing is going on that you need to worry any more about.
I had to take things one day at a time, and one thing at a time then, otherwise I would have not done it either, but I felt that things were getting worse and worse, day by day and I didn't really have a choice.
Hang in there Retired mom and know that I am thinking of you.
Sandi