There are many posts about people upgrading to Windows 10 and then having compatibility problems such as a black screen or no sound etc. I will try to describe how to make the upgrade to Windows 10 as problem-free as possible.
First rule for upgrading to a new operating system is to download all the drivers we will need beforehand. Worst case (except for BSOD) is that the network card no longer works, so you cannot download new drivers for everything else! So we visit the manufacturer's site and download all newest drivers. If there are not Windows 10 drivers, download latest Windows 8 or 7 drivers or at least Vista drivers for rather old devices. Make sure you download the correct version 32-bit or 64-bit, depending on your current system. For getting the newest drivers I recommend going directly to OEM's site (Intel, nVidia, AMD, Realtek and so on) because your computer manufacturer may have not made them available on their support site yet.
Just a warning here for nVidia cards: From Forceware 340.xx and later the CUDA component has changed so old applications cannot detect your card's CUDA capabilities and you won't have hardware acceleration in video editing and other applications. So anyone having a not-so-recent nVidia card, such as my GeForce 620, download an earlier Windows 8.1 version, I'm using 335.23, or you won't have CUDA support in applications! Yes, those versions work OK in Windows 10, I have this version installed now and I'm running Windows 10 Pro 64-bit. Latest driver has almost nothing to offer for our older GeForce cards anyway.
Now that we have downloaded all the drivers, we must do some more things before starting the upgrade. We have to disable or even uninstall the antivirus so it won't interfere. When I say disable, not for some minutes or until next restart! Disable it completely. Windows Setup will restart the computer several times before it is complete and surely you don't want the antivirus to block anything and cause issues!
Last step is to disable any service and application at startup that it not very important. We can re-enable them after the upgrade. So press Win+R to open a Run dialog and execute msconfig. Go to Services tab and click to hide all Microsoft services. Now uncheck any service that is not very important (practically almost everything that is not part of a device driver). Then click Apply. Now go to Startup tab and click Open task manager. Disable any non-important applications such as google update, Acrotray etc.
Start the upgrade process. Wait for the report. If you have to uninstall an incompatible application, do so and restart it. When at the first Window you should see two options with a checkbox. The first is to search for updates during setup. UNCHECK THE BOX NOW! If you read it carefully, you will see that it will try to replace your drivers with newer from Windows Update! This is what causes all compatibility issues! I would also disconnect from the internet during the upgrade to be on the safe side.
After successfully upgrading to Windows 10, create a restore point so you can return there if you need to. Click on the search icon and type Restore to find the link. Then install all new drivers you downloaded before and restart the computer as necessary. Since other drivers depend on it, start from the chipset driver and then graphics. After you have installed all drivers and you have no problems, create a restore point again. Now you can safely connect to the internet to activate Windows 10. Also run msconfig again and enable the services and applications you disabled before the upgrade. It is a good opportunity to look at them carefully and keep useless ones disabled so your computer has fewer delays during startup. Last step is to reinstall or enable your antivirus.
I hope this advices will help you upgrade to Windows 10 without any problems.