If you perform a Clean Install of Windows meaning boot and install Windows, there is a menu ask for partition, if you don't modify the partition which Windows has been installed and chose to install Windows on the same partition as the previous version of Windows, then it will show you a message that your files will be placed in a folder called Windows.old and when you install your Windows, you may open the folder and retrieve your files. However, you have to reinstall your applications. In case, you format drives , then you will lose your data.
Normally, if you change the hard drive only, your activation won't run into problem and should work fine. The real problem is when you change motherboard or you have some other major hardware change and that also depends on your license. However, incase if you change hard drive and run into activation issue, you may look for solution in this forum or if you won't find it, you may ask a new question.