The W10 upgrade is similar to any other upgrade installation. IMO it's actually a tiny bit smoother - but I believe that that's due to what Microsoft has learned with the upgrade process on previous OS's
The critical factors here are:
- the age of the installation (how long has Windows been installed on this hardware)
- how well used the installation is (did you use this daily, or was it put in a closet and left until now)
- how much stuff you have installed on it, and how well updated all of that stuff is.
In short, the older and more well used the system is, the more likely it is that you'll experience problems when installing the W10 update.
There's several different ways to help offset this:
- backup your data before starting.
- create a system image (in case you want to go back to the previous OS). Sometimes the revert process will fail, leaving you without an OS
- capture your W7/8/8.1 product key before starting
- ensure that the W7/8/8.1 system is activated (and with W8/8.1, check to be sure it's permanently activated on Microsoft's servers)
- set a system restore point before starting.
- download the USB W10 drive to make a clean install (if needed) ( https://www.microsoft.com/en-us/software-download/windows10/ ). It also has repair tools on it, so I suggest downloading it first (you may want to wait until August 2nd for the Anniversary Update version).
- as soon as you get into W10, ensure that the system is activated (that way you won't have problems if you have to reinstall)
If the update fails, post back for more detailed instructions.
Include info on how far the installation went before it failed, and what happened after the failure.