Sooperbohl
After upgrading, is there anything you have to re-tweek or turn off or turn off again? Anything to be cautious about?

 
A W10 update may restore defaults that you changed. For example I keep the "HD Audio" codec installed by graphic cards turned off because it impacts performance. Sometimes after an update, W10 will be "helpful" and turn it back on again.
 
 
 

If upgrading both Sonar 8.5 and W7 (to W10), which one should I upgrade first? Or would it matter?

I would update the OS before installing a new program if given the choice. 

As said the only reason not to upgrade would be drivers. I don't believe this is an issue from 8.5 to 10. But it certainly is from W7 to W10. A few people I know lost drivers for Laptops extra stuff like web cams and card readers when they upgraded from 7 to 10. My wife is seriously thinking of reverting back to 7 as there are no solutions other than using outboard gear.  
I have them all running right now, 7, 8.5 and 10 and all work fine, I even still have XP on 3 old laptops and a desktop that refuses to die. At work ( huge health authority)  we use W7 and that won't change soon.
Hell, I'm even getting used to Android and Apple on a few phones and tablets devices and I really like that too. There just different that's all. I would never say one was better than the other, they each take getting used too and then tweaking to perfection, Apple seems to need the least amount of tweaks.  

Laptops always suffer more during an upgrade. The problem lies in the custom integrated peripherals used which require - custom drivers. These drivers sometimes survive during an upgrade to the *next* major OS release, but usually not two releases.
 
For example, I redid a bricked up laptop originally running Vista to Windows 7, and then to Windows 10. Windows 10 did not have the proper driver for display, leaving it at a less than optimal display size, and also lacked audio support. A look at the Mfrs site, HP, showed that the only supported drivers were for Vista, it's original shipped OS. I could manage an unsupported and ancient beta nvidia driver to correct the screen, but the sound system was a no go. I rolled it back to Windows 7 and that is where it sits. It's not economically viable at this point to chase it down further - it's an 8 y/o laptop.
 
A counter case is my 5 y/o gateway laptop which shipped with windows 7; I upgraded it to Win 10 w/o a hitch.

Anderton

elegentdrum
I have an SSL alpha link and MX4 sound card (Soundscape) that only officially support Windows 7. I'm also very afraid of the upgrade for fear of needing to re-enter, verify, and update many 3rd party programs. I have about 50 program codes I don't want to have to go through again. If something does not work with Windows 10, going backwards will require a re-format and go through the install process again........days of work.
 
So I will resist the upgrade for as long as possible.

FWIW you don't have to wipe your drive to upgrade to Windows 10. I did an in-place update and aside from re-authorizing Addictive Drums and reloading the TASCAM and Roland interface drivers, that was it. Anything that was tied to the hard drive didn't care there was a different OS, and of course anything with iLok didn't care. Upgrading to Windows 10 was by far the most painless Windows upgrade I've ever done. It was easier than going from Mavericks to El Capitan on the Mac.
 
Of course you should do what you want to do, all I can describe is my own experience.

 That's the best way to go. Just don't use Windows F'Update.  2 systems upgraded with an .iso.  3rd one failed with F'Update.   Make an image of both then revert back if necessary.
  All of my legacy M-Audio stuff works in W10. It pulls up anything needed from W7.   In my failed update system W10 used beta 64 drivers for my Terratec EWX2496.  So IMO W10 has excellent legacy support.
 
 
 
 

i upgraded 3 system via the in place windows update, and 1 clean install with a retail copy of windows 10 pro. no difference between any, they all run perfectly and the upgrades went without a hitch. kudos to microsoft for that. i wouldn't hesitate doing an in place upgrade via windows update if i had it all over again. but that is just my experience.

The Grim
i upgraded 3 system via the in place windows update, and 1 clean install with a retail copy of windows 10 pro. no difference between any, they all run perfectly and the upgrades went without a hitch. kudos to microsoft for that. i wouldn't hesitate doing an in place upgrade via windows update if i had it all over again. but that is just my experience.

 
I've got an old 8Gb USB stick that I loaded a bootable versions of Windows 10 on. (You can pickup 8Gb USB sticks at the Dollar Store these days). That and a copy of Sysinternals tools and I'm ready to make some extra cash. There's gajillions of folks that need help and resent forking $200 over to the Geek Squad.

On second thought, it makes sense to me to just upgrade Sonar. If it works fine on W7, with all issues resolved, I don't see any great benefit from going to 10. If anything, I would upgrade to 8.1 before 10. By 2023, W8 EOL, I expect there will be a Linux DAW I can use, and I will say bye bye to Microsoft.

SuperG
 
 I've got an old 8Gb USB stick that I loaded a bootable versions of Windows 10 on. (You can pickup 8Gb USB sticks at the Dollar Store these days). That and a copy of Sysinternals tools and I'm ready to make some extra cash. There's gajillions of folks that need help and resent forking $200 over to the Geek Squad.

?
 
i ran that through several translators but they all came up with 'unknown dialect'