OEM code is locked to motherboard as was mentioned. You can change anything else you want.
Keep in mind that the storage controller(s) on an OEM machine may not be set to AHCI, and if you are adding an SSD, you will definitely want that controller in AHCI mode, necessitating a windows reinstall if that SSD is going to be the OS drive.
If you are already in AHCI mode for that storage controller, the easiest thing to do if you want to swap your boot drive to an SSD would be to clone the existing drive in the system to the SSD using some free cloning software and then wiping that HDD and using it as a storage drive.
If all you want to do is add some additional hard drive storage, all you need to do is hook the drives up and mount them in disk management in windows and you're good to go.