Hey, guys! So I currently own a MacBook Pro 15" from late 2011 (w/ the optical drive). It's got a 2.2 GHZ Intel Core i7 with 8GB of memory and an Intel HD Graphics card. I've also got a 1TB SATA Disk. I'm wanting to switch to a lighter laptop, but still want to save my money. So I was considering just buying a used or refurbished 15" with a retina display (2012 or newer). Then I'd switch over my hard drive and whichever other parts I can replace. It sounds simple, but I know little to nothing about replacing hardware, and in a Mac no less. So my question is, what parts can I replace in the new computer? And which ones should I replace? Hard drive CPU/ Processor Logic board/ Mother board RAM Graphic card Battery And if you know, how can I go about buying the new computer w/out having to pay extra for the parts I already have? Where do I look?