Most SSD manufacturers make migration simple. Often times you can simply clone the OS onto the SSD and swap the drives. If you're on windows 8, UEFI bios stores the keys so there won't be any need to deactivate. Windows 7 and earlier may throw tantrums but whatever error code should be google fixable. If you plan on using an external caddy, you can keep the boot sector, but if you plan on having both drives internally connected, you may want to make sure only one of the storage media has a boot sector selected or else you may have to boot with boot manager every time. You can use the diskpart command in windows to clean the HDD. *Be warned, diskpart can and will delete entire disks if used improperly, read tutorials carefully and make sure the selected disk is the one you want to clean*.
You may run into issues with drivers and such. There are also alot of optimizations that windows 8 doesn't handle automatically. Google is your friend.