If I buy Sonar Artist, can I use the plug-ins I got in Sonar 8.5? Such as LP 64 Equalizer, LP 64 Multiband, and Tube Leveler? And the full Dimension Pro?

AFAIK there should be no issue since you bought those plugins with 8.5 and are free to use them in any software that supports VSTs. Sounds like you're considering the same thing as I have done in the past. I was on X1 Producer and was happy with the plugins there. So I thought about downgrading to Artist instead of moving to Platinum. In the end I did go for Platinum when there was a big discount on Platinum around Black Friday.
 
Also: Cakewalk confirmed to me that downgrading would still allow me later on to upgrade my old X1 Producer to Platinum if I wanted to. I wanted to be sure since X1 Producer -> Platinum is cheaper than Artist -> Platinum. 
 
EDIT: here's my old thread that also kinda covers your plugin question: http://forum.cakewalk.com...-m3153645.aspx#3153645
 

Thanks for that info, Rocsee. I doubt that I will ever get Splat, it's just way more stuff than I need. I've been happy with 8.5, except that it's no longer as stable as it was, and certain vstis don't work right in it (crash, take too long to load). I just bought Reaper and am happy on the whole with it, but I need to finish my current project, a large orchestral piece. It's taking too long in Reaper as I'm simultaneously having to learn a new DAW. Also, 8.5 can only export in midi, not xml. So it seems like a good idea to do this while I transition to Reaper. Should make exporting my projects easier and more reliable (my current one somehow got corrupted in the export/import). And for 49.00, the price is not an obstacle.

michael diemer
Thanks for that info, Rocsee. I doubt that I will ever get Splat, it's just way more stuff than I need. I've been happy with 8.5, except that it's no longer as stable as it was, and certain vstis don't work right in it (crash, take too long to load). I just bought Reaper and am happy on the whole with it, but I need to finish my current project, a large orchestral piece. It's taking too long in Reaper as I'm simultaneously having to learn a new DAW. Also, 8.5 can only export in midi, not xml. So it seems like a good idea to do this while I transition to Reaper. Should make exporting my projects easier and more reliable (my current one somehow got corrupted in the export/import). And for 49.00, the price is not an obstacle.

While it is a jump.  You might want to seriously consider the jump to lifetime SPLAT at $170 (audiodeluxe.com when you put it in your cart).  
 
8.5 to the Current Sonar is going to require some learning curve as the interface while somewhat familiar is different.  
To me, Reaper just doesn't have the same user interface as Sonar, and that is huge.  
 
If you use the LP stuff, the new version kind of make the old ones look like toys...though they are very processor heavy in full Linear Mode (you can use Non-linear mode also).  
 
I was on 8.5 Prod, and tried reaper to save money and wanted to see what the "slim down" program was like.  Now that I have Sonar's skylight, Reaper just seems archaic....and so does 8.5.  (Though I don't do any heavy midi manipulation).  
 
Just something to think about.  

Actually, I've been demo-ing Sonar Producer, so it's not that unfamiliar. With my workflow being simple and plug-in use minimal, it's not that big a jump.
 
As for Platinum, or even Producer: I do midi exclusively. I have never used Melodyne or Pro Channel. So why pay for them?