Is it possible to upgrade the SSD in a 15" Macbook Pro (Late 2013)

OWC has a replacement flash storage http://www.macsales.com/. However, The new retinas aren't meant to be modified after purchase, so this will void your warranty.

Yes, but if you do it yourself it will void the warranty. Check this site: http://www.everymac.com

If hardware fails and it's not caused by user abuse or accident then it is very helpful to have. Why not purchase an external storage device?

Is the warranty really good for anything? I've never really had to use it.

My opinion: Yes it is. You pay for one year of warranty coverage when you buy a Mac. It's in the purchase price.

I have used it many times. In fact I always buy extended Apple Care within the first year so I am covered for three full years.

From OWA Customer support this Am 11/6/14 ..

You are now chatting with OWC Rebecca R (Customer Service Representative) - Sales and Customer Service (24/7)

07:15OWC Rebecca R:Thank you for contacting OWC. One moment please, while I look into your question.

07:15OWC Rebecca R:I will be happy to assist you with your inquiry Robert Martin.

07:16OWC Rebecca R:I am sorry but we do not offer SSDs for the late-2013/2014 retina Macbook Pros yet. There are no part numbers yet.

07:16Robert Martin:Do you know when it will be offered ?

07:16OWC Rebecca R:Sorry no eta yet.

07:16Robert Martin:ok thanks.

Not correct, not even now - May 2015. Not looking too good for this particular model, it would seem.

External storage is not exactly the same, especially on a laptop.