I think that the default themes are there to be used. These themes are coded to the standards that should be used and excellent example for new theme authors to learn from. They are also all under the GNU General Public License. I would really think that these are excellent basis to work from when developing a new theme. Why reinvent the wheel when it is already there
I have recently done the same exact thing. It is quite tricky, as it not just simply a mass search en replace operation. You have to methodically check and replace instances. The easiest way to start here is with the localized strings. Simply use a program like Notepad++ and replace 'twnentyfourteen' with 'my-theme'
The safest to go next is to take a string and replace twentyfourteen, like @since Twenty Fourteen 1.0 replace with @since My Theme 1.0.
You need to constantly check that you don’t break anything. and if I can give you advice here, do one by one file, Notepad++ have a mass find and replace function. Don’t use that. Take one file at a time and replace all instance of twentyfourteen, check and recheck and move to the next template. Grueling task, but worthwhile