If you keep your motherboard you have no problem switching other components such as SSD. Your system is already a Windows 10 Pro Device. During clean installation with Windows 10 Pro TH2 installation media your Windows 8.1 Pro OEM key will be automatically input from the UEFI BIOS and Windows 10 Pro TH2 will automatically reactivate when your systems online.
Hello there, I have searched the internet with the question I have before I do the upgrade and the re-installation of windows. But the posts and solutions are all scattered. I really want to clear this for myself once and for all if possible.  First, the hardware that I have purchased for this upgrade: 1. Samsung 850 EVO 120GB mSATA SSD (to replace the 32 GB factory default one) 2. Western Digital Black 750GB Performance Mobile HDD (7200RPM, 2.5 Inch) 3. Intel(R) Dual Band Wireless-AC 7260 (802.11ac, 2x2 Wi-Fi, Bluetooth) 4. Crucial Ballistix Sport SODIMM 16GB(2x8GB) PC3-14900 CL10 at 1.35V 204-Pin Memory BTW, My Dell XPS 15 L521x has a intel core i7-3632QM and a nVIDIA GeForce GT 640M. It also comes with 32 mSATA SSD as cache for the WDC 1TB HDD.  My goal: 1. Install/reinstall Windows 10 on 120GB mSATA SSD and free 500MB give or take for system reserved partition. 2. Use the remaining space as C: Drive. 3. Use WD Black 2.5 inch 750GB HHD for regular storage 4. Remove and then install the Intel Dual Band wireless Card for a better Wi-Fi compatiability for most of dual band routers out there. (The old factory default one, well, is not great to say the least) 5. Remove and then install the crucial RAM (compatiable with L521x) is to bring my laptop's memory up to date (Maybe a little future proof ;D).  Prior to start the upgrade, I know that I have to do the following steps: 1. Update my BIOS/UEFI. (I have done that) 2. Disable Accelerated from Intel Rapid Start Technology(IRST) GUI (My Laptop keeps prompting that "your system does not have intel repaid start technology enabled"). 3. Change Intel Smart Respond Technology(ISRT) to AHCI in BIOS 4. Backup my Files 5. Backup my Windows 10 Pro Activation ( How to exactly? The Win10 Pro is upgraded from Windows 8.1 Pro with Dell OEM)  Now my question is what else do I need to do or what have I missed so that I won't run into any errors or problems if possible. Also, I have heard or seen something about the Windows OEM version. If I switch the hardware, the backup activation file or recovery disk won't recognize my device. Is this true? If necessary, I can buy a new retail version of Win10 Pro. But, do I really have to do that, and is there any not-so-complicated work around?  Many thanks, Dean
How to upgrade the hardware of Dell XPS (L521x) and then to run a clean 
reinstallation of Windows 10 Pro