The following is a quote from a ZD Net article regarding activation of Windows 10, but also addresses what components can be changed without ramifications, and what still can't be changed without activation problems with OEM computers.
"For more than a decade, one of the keys that Microsoft's activation servers have relied on is a unique ID, which is based on a hash of your hardware. Microsoft doesn't provide details of how it calculates that hardware hash, but upgrades of system components such as a video card or a hard drive won't normally trigger a reactivation. If that happens, a quick call to the activation line will resolve the issue, often without any human contact required, in minutes.
The one exception is a motherboard replacement, which will inevitably cause the Software Licensing Management utility to recognize the device as a new PC and require reactivation, typically over the phone. A motherboard upgrade, even if you reuse storage, video, memory, and a case, is considered a new PC. In that case, if the underlying Windows license is from a retail copy, that license can be transferred. If you are upgrading (and not replacing) a motherboard on an OEM PC that was sold with Windows preinstalled, the license agreement prevents the license from being transferred."
As for activation of Windows 10, once a successful installation of the operating system is completed the operating system will automatically activate in the future.
Edited by dc3, 12 October 2016 - 11:07 AM.