I don't think any of these commercial panels got worse
with a successive generation.
Don't know if you've seen the thread by a member called "IamaNoobieCheez" where he posts lots of hi-res shots of his 7UY vs. his 9UY, but it might be helpful:http://www.avsforum.com/avs-vb/showthread.php?t=733641
In this thread, you can see that the 7UY vs. 9UY picture quality (as you surmise) is very, very similar. The biggest improvements are to black level and overall color accuracy.
If you read CNET's reviews (they've reviewed the 50" panny pro model for several generations) they also agree that year-to-year improvements are fairly small since 7th gen or so.
Black level for 7uy/8uk was a slight improvement over earlier models I think. I do know that the black levels did improve a lot between 8UK and 9UK, with contrast ratio going from 3000:1 to 10000:1.
I would like to get one on the cheap, used, refurbished, clearance old stock. What have you.
The problem is that with older models you can never find them for sale from actual vendors at non-ridiculous prices. The only places you find out-of-production plasma models are these shady internet vendors who are still charging like $3000 for a 3-year-old 50" model or whatever.
So if your thought process is "I could save money by finding an older 7UY and not sacrifice much in picture quality compared to a 10UK" then you are mostly right. Shoot, my parents have a 42" 5UY ED panel and it looks freaking great, with HD material you wouldn't even know it was an ED panel unless you were sitting within 10 feet.
The problem is that they aren't available anywhere so you will just have to be patient and troll the used market. I see older-gen Panny pro models coming up used on craigslist and ebay every now and then, but it's pretty rare, and I'd be a little wary of buying one used that was in a commercial application.
Another issue is that the price has just dropped like a rock on plasmas, changing the whole used/new value proposition -- in other words, you may be able to find a 42" ED panel (a 6uy or 7uy) on craigslist for $500 or so... but a 42" hi-def 10UKA with dual HDMI inputs, superior resolution, black level, and overall PQ, with a free 2-year manufacturer's in-home warranty, is $859 at forum sponsor Visual Apex! I'd take the new model every time in this situation, that's a lot of added value (especially the peace of mind of warranty and the convenience of built-in HDMI inputs).
If your money is really tight and those extra $300-400 are really a big deal, then you should be patient and bide your time and wait for a 50" model. I think that's a better value proposition if someone is selling a 50" 6UY or 7UY for the $600-700 range (which I do see occasionally on craigslist). The resolution jump to a 50" 1366x768 is pretty big.