Increasing Lower Back Pain?

Good Morning
I will get straight to it.  I'm a 46 year old man living in Tasmania.  In 2003 (Jan) I had a lower back fusion (S1-L5) with instumentation while living in Orlando,FL the surgery was a result of being born with a spondyolythesis which had progressed to a stage 3 and was only continuing to worsen as Im a tradesman plumber and worked in construction at the time.
My Neurosurgeon when we met had 1 look at my X-Rays and said I was headed for this moment in time from the day I was born, Dr.Baker and I got along very well and I was booked in for the surgery.
Pre-surgery he ran thru' the procedure with me and said that along with the instrmentation he would be in-certing "cages" between S1-L5.
Post surgery he told me that my vertebre had too much deterioration for the placement of cages so he applied the bone cement along with the screws and 2 vertical plates.
I was off work for 8 mth's and wore the 'turtle-shell' along with an electrical device that was prescribed as I was a smoker at the time and was told that the device(belt with round loops) would help getting blood flow thru' to the smallest of capilleries as nicotine had a negitive effect in this case.  The belt was set up automatically and I wore it for 3 hr's twice a day.
After 8 mth's my x-rays and other were all good with the cement etc,,, setting perfectly.  In fact my surgeon asked if he could use my case on his lecture tours as it was such a good example of that surgery and conclusion.
He did caution me that as I had lived a very active physical life and the degeneration of my condition at 40 yr's old it may take up to 6 yr's for my spinal cord to 're-wire' itself and to live pain free.
I was pain free after the 1st 2 months post op' it was great no drugs and no pain and I spent my time gently swimming ,walking and enjoying life.
I went back to work plumbing which was ok as long as i did nothing stupid.
It's now Sept' 2009 and my pain has returned down my left side and lower back for a few yr's now and Iv'e just had an mri which shows the start of a protruding disc at L4 due to the extra workload on that area of my back and I was told that this was a possibility.  The mri was ordered by my specialist due to the increase in my pain and discomfort.
So at this point my pain management Dr. (specialist) has said that I will need a stronger pain med so I will be going from 15mg 'Norspan' patch to a 25mg Fentanyl patch and my Doc' said to me this week that we will work on finding me a pain med that will be comfortable.
Im in a carrer change as the heavy work of construction is just not suited to me anymore but I still do smaller type jobs when I have to, eg; bathroom renovations etc,,,
My questions are 1. what is the best way to get off the Nospan patch and transfer to the Fentanyl patch? does 1 cancel out the other?
                         2. why after 6 years is this back pain coming back with such force as the mri shows that the area of the surgery is still in great shape?

I can still do everything in my life but Im lost on why the pain is coming back.  Iv'e had lower back injections(blocks) but they did not seem to do much if anything.  Also tried Gabapentin but I experienced nasty side effects.
Any thoughts on what could be going on would be greatly appreciated.

Regards, Nosweat00