I'm sorry, but there is no way to transfer your file from the desktop platform to the online platform.  (You can do the reverse, but not what you're asking to do).

If you decide to return to the online platform, you'll just have to begin your 2016 return (next spring) by referring to your copy of your 2015 return.

But....(my personal opinion here), you may want to consider staying with the desktop platform.  The pros:  All of your information is stored on your personal computer; you and only you have access to the returns.  Your returns are never saved on TurboTax servers.  Additionally, the desktop program allows you access to the "Forms Mode"...you can see your actual form as they are being prepared before you file.