Hardware fix was easy, Software issues not so much.
When I bought the MacBook Pro in mid 2009, I only got 128 GB of memory and 4 GB of RAM. Seven years later, and I've run out of space and my RAM just isn't fast enough, so I decided to double my capacity to 256 GB of storage and 8 GB of RAM.
Installing the new hardware was so easy, extremely easy in fact while using your visual instructions, but software installation on the new hard drive was an incredibly difficult process. Keep in mind, that when you take out your original hard drive there's a possibility that disk errors could occur, or if you were as close to empty space as I was, your drive might not reboot again. This was my case. And do yourself a favor, make a complete full backup of your hard drive before removing it. I am so glad that I did. Also, when you reinstall the OS X, make sure that you understand that all of those updates and new releases will have to be reinstalled as well. Because I was installing snow leopard 10.6.8, which is 7 years old at this point, it created a real mess where some of my applications either didn't work or were found to be incomplete. It wasn't until I reinstalled the OS X El Capitan that everything fell back into place. One major recommendation I would have is to make a bootable flash drive with OS X El Capitan so that if you ever need to replace another hard drive, you will be able to install the most up-to-date version of OS X onto that hard drive. It literally took me two days to get my software issue fixed and the equivalent of about 10 hours worth of research, updates, and downloading to get me back to 100%. Oh, and finally make sure you that you do another complete backup of everything you've done. I really hope that the information I provided here will help someone as it may have helped me.
Read above for my recommendations.