I have to be honest - first time I've ever heard of your situation.
To my knowledge Series 5 is budget, Series 6 midrange and Series 7/8 is premium. In fact the only difference between 7 and 8 in the H is the curve in 8, otherwise they're the same.
Also never heard of them rebranding different Series local and abroad. Are you sure you don't have a Samsong from the side of the road?
I've yet to see a plasma be *overall* better than my Series 7, *although* in honesty I haven't seen a Premium plasma. I see screen uniformity (though seldom to I watch black screens. I tend to watch a movie or turn off the TV) and can occasionally notice motion.
That said, image retention is the absolute worst thing about plasma and the reason I sold mine. I cannot cope with even a few seconds of the previous screen. To me the trade between an uneven black screen and two merged scenes for a few seconds is well worth it.
Now, I need to find someone like you that will accept budget obsolete technology for their high end screen to swap. Tag, although he will publicly say it, will not swap