My SONAR "end of life" contingency plan involves doing a fresh W10 install on a new C Drive.
However, I'd like to maintain my *current* C drive so I can continue to work on projects.
( Swap new drive, do my installs, swap back to current drive when it's time to work. Switch to new drive when all is configured and good...then clone the new drive and stick it on a shelf if ever needed)
 
I've never done this dance with a Win10 OS. Previous Win installs had a 30 day grace period before having to register the OS with the mothership. This always gave me time to do installs but still work on current drive until the new one was working well.
 
Anyone who's done this, please advise on any gotcha's to this plan.
( I won't do many 3rd party plug in auths until the drive is pretty much ready to go, so I think I will sidestep those issues.)

If you're going to be working with a dual-boot setup (even with one boot drive at a time):
Make sure you have "Turn On Fast Startup" disabled.
 
Otherwise, you'll be fine
 

I have a dual boot Win 10 on a laptop that I bought recently.  It has the Dell OEM Win 10 on the original M.2 SSD and I installed another Win 10 on a separate SSD.  When I want to use the Dell version I can boot to that and when I want to use Sonar (and now also Studio One 3) I boot to the separate SSD installation.   As I only use the laptop for on-site recordings the Dell installation is set as the default OS on boot up.  
 
I don't think fast boot makes any difference.  On my desktop, I have dual boot with Win 10 and the Insider preview version Win 10.  When the preview version updates it always sets fast boot on and sets the preview as the default dual boot OS, but I still get the dual boot option window.  I've been turning fast boot off but thinking about it as I write this it doesn't make any difference, at least on my system. 
 

Thank you both for the replies.
For others doing similar, there is indeed an option to skip the MS login and activation process and continue the install process as with previous OS installs.
 
Once I have a nice clean W10 install with a fully current SONAR install, I will clone that for any future projects that come back to haunt me with SONAR files that need to be accessed.
 
Once fully populated ( SONAR, plugs, configuration, as well as Presonus SOP) will then use that as my new system drive *and* clone that version so I have a backup image of my new current working drive. 
 
That should cover me for current system as I transition to Presonus, while allowing access to SONAR as needed.

I too will be transitioning to Studio One 3 Pro.  I have both Sonar and Studio One backed up on flash drives.  As Sonar does everything I need now it will be a "leisurely" (no hurry) move to Studio One.  I purchased the Groove 3 Studio One 3 Explained course and will start going through them.  
 
 
 

Exactly my plan as well. By the time I make the full switch to SOP I will be fast and fluent. No real hurry if I plan the transition properly. Part of my "learning curve" will be transferring very recent client projects to SOP and that will solidify my process for dealing with past projects that might resurface. 
There's a lot I like about SOP so it doesn't feel like the daunting prospect it did even a week ago.
 
If I run into any snags in this master plan I will update here :)

fireberd
I don't think fast boot makes any difference.  

 
Leaving the "Enable Fast Startup" option enabled can/does cause major data corruption (even if the dual-boot is across two totally separate/isolated drives).
We've experienced this first-hand.
With a modern SSD as boot drive, the machine will start fast without using a Hybernate file.  

duly noted ( bad pun somewhat intended)..thanks Jim.

I don't use it, I always disable it,  but mentioned as it seems to still work OK.  As it is only the Insider Preview version and not the Production system, even if I left it enabled it wouldn't have any affect on the Production system or DAW operation.
 
 

My previous post should read:
 
 
When dual-booting the OS, leaving the "Enable Fast Startup" option enabled can/does cause major data corruption (even if the dual-boot is across two totally separate/isolated drives).
We've experienced this first-hand.
With a modern SSD as boot drive, the machine will start fast without using a Hybernate file.