I called Microsoft, well had them call me back before I updated my Win 7 Pro x64 retail, to Win 10 Pro last week, as I had the same concerns.
The guy I spoke to assured me that I can always go back to Win 7 if its a retail version, even after one month, or the 29th July next year, and basically stated that they cannot change the contract conditions of the original Win 7 pro x64 purchase.
After reading so much about this the last few days, I was unsure again so this time chatted to a Microsoft agent online.
The agent made it a lot clearer, and it doesn't sound good for the long run if you upgraded from a retail licence and want to stick with Win 10.
I selected all text and copy pasted it to my notepad before ending the chat, so can paste it here minus my product key if needed.
Basically, or not so basically...
I asked lots of questions, from upgrading your OS drive to building a completely new system, and what happens to the retail licence etc.
If you upgraded from a Retail version of 7 or 8/8.1 you can go back to win 7 in my case any time you want, and it should activate without any problems, even after the 29th July next year. Note: "Go back", not keep using windows 10.
If you upgrade your primary O/S drive, the agent recommended that you install windows 7 first, activate it, install all updates, then update to Windows 10, then you can clean install using the Win 10 MediaCreationTool64/32 if required. He said this is the best way to ensure there are no activation problems.
If after updating to Win 10 you create a clean install dvd/usb and try to use it on the a newly purchased SSD/HDD to replace the OS drive, it may not activate, but he suggested to try it first anyway, if not install Windows 7/8 first activated with all updates, then update to Win 10...
I told the agent that I regularly upgrade hardware, and was told, as long as you have a retail licence you can make any changes you like, even the MoBo, and if at any point windows says there have been too many hardware changes and to contact Microsoft, your retail Win 7 Retail key can be used over the phone to activate it again.
I said that I will probably build a completely new system next year, and can I just transfer my licence to that system?...
This is were the bad news come in...
As long as I build the new system before the 29th July 2016, and my original Windows 7 is retail, I should be able to do the same thing, do a clean install of Win 7, activate, carry out all updates,, then update to Win 10, and create a clean install.
The agent specified the 29th July 2016 on this question, so any major changes after that date, and you will need to purchase a windows 10 licence. But from what I understood, you will always be able to go back to your retail Win 7/8 even after this date, but not use it to get Win 10 for free.
This to me sounds reasonable, after all we are all getting this as a free upgrade, if you don't want to use it, go back to Win 7/8 until its end of life.
You are not loosing your Win 7/8 licence, you just wont be able to use the key to continue using the free upgrade to Win 10 after the 29th Jjuly if any major hardware changes are made, but you wont be without an OS, you can either roll back, or pay for a new Win 10 licence.
It all sounds reasonable to me at the moment, but I will be upgrading my OS SSD soon, as my aging 256GB is near full, so will see if its as straight forward as I have been told, although I don’t call having to install the OS 3 times straight forward if you want a clean install….
If you love Win 10 and are not bothered about going back to Win7/8 in the future, it wont matter, and you know if you make to many changes you may end up having to pay for a new licence after 29th July next year, and if you are planning to build a new rig, have a retail licence and want to take advantage of the free upgrade to Win 10, then you better do it before July 29th next year, if not, you'll have to pay for it, or go back/ continue using your retail Win 7/8.
A few people have mentioned Nvidia driver problems, I had the same thing, drivers didn’t install properly, after a reboot drivers looked installed OK in the device manager, but system kept locking up apps after a few minutes, and the downloaded Win 10 Nvidia driver would not install. Used the DDU uninstaller (must be extracted and used in safe mode), tried the Nvidia driver install again, worked, and all problems were solved, and Win 10 is now running perfectly with all the apps I use, 3ds max 2016, Paintshop, Photoshop, Mudbox, After-effects etc, no problems so far, and rendering with 4 GPU’s in iray works fine.
Had a problem getting the force enable Gen 3 to work on my X79 MoBo. It no longer had any affect after reboot, and checking in GPU-Z. Problem was solved by copying the force enable gen3 exe to the main C: drive directory, and running the force enable gen 3 executable in the command prompt as Administrator. All 4 GPU’s listed, and after reboot, Gen3 works. You have to change the directory to C: first by typing cd C/: then type in force-enable-gen3.exe . A security warning will pop up, but just accept it, Done. You have Gen3 enabled next time you reboot. No need to mess around in the registry
I had micro stutter after upgrading from two GPU’s to quad sli in windows 7 Pro on a few games like BioShock infinite and Meto Last Light, tried every way possible to get rid of it, but never could, luckily most games ran fine, and the micro stutter was very intermittent.
Tried Bioshock infinite and Metro in win10 in Quad sli with GTX titans, and the stutter is completely gone, runs very smooth even with Vsync off, in fact sickeningly smooth, so that’s one thing I am happy about with Win 10. Apart from that it seems to be just like Win7 but with all the cr*p I don’t like about win 8 shoved into the start menu. It also feels a bit more polished than its predecessors, and 100% stable so far since the Nvidia driver problem was fixed. (touch laminated wood!)
Not sure if all this reassures anyone, it seems one person says this, another says that, one agent from MS says this another says something completely different, but the way I look at it, is you’re getting this for free, and if the agent I spoke to is right, you will always have your Win 7/8 licence to go back to, that is until it reaches its end of life, then you have to upgrade eventually, so why not take advantage of the free offer, and if you are always building new systems, don’t think of the free win10 upgrade as an extension of your existing Win 7/8 licence, think of it as a one year lease, after that, you know you’ll need to purchase a new licence if you make too many hardware changes like the MoBo and want to continue with Win 10. Don't like the sound of that? Go back to Win 7/8.