I have a base rMBP15 that I might replace with a base rMBP13. I've used rMBP15 for about 5 months and it serves my purpose very well (mostly office work - Office for Mac 2011 Excel, Outlook, Powerpoint, Adobe Acrobat while running Windows 7 via Parallels, web browsing and watching video files via VLC). I like it but I just want to try something else. I have a chance to buy a new rMBP13 for a very good price so am thinking about selling the rMBP15 and replace it with rMBP13, but my only concern is the base rMBP only has 128GB SSD which will be not enough (I'm using about 200GB now). Will I feel any slowness from using rMBP13 based on my usage? How difficult and costly is it to replace the 128GB to 256GB SSD in rMBP13?