Depends on the files. If they are loose files like music or photos or documents, sure, no worries.
The problems arise with installed files like programs etc as Windows installer has records of registry links and associations etc. Those will not 'transfer' with a simple swap, but must be reinstalled from scratch in the new OS.
Can't even keep the old OS as such as its tied to the old motherboard and old drivers etc, so you'll need a clean, fresh install there too. You can reuse the old key, IF it's a retail key and not a Dell or HP or other 3rd party key.
OS is free, just dl directly from MS, get the version you already have, MS has record of the key (it's digital) unless you have a physical copy. Just log into your MS account and...