Yeah I tried this past weekend - and it was a dismal failure. I ended up creating like 100 UEFI entries on the drive... Never really got it too boot into Sierra/complete the install. When booting back into El Capitan - it was broken, no audio, spotty internet etc.
Granted I have no real experience doing this, as I just followed Jerivalu's guide getting El Capitan working (which worked 100% with 0 issues...). Compounding my stupidity, I didn't even to the Carbon Copy Cloner backkup properly - I backed up files, not an image. So all I can do is restore individual files/programs...
So, I formatted the drive before work this morning, and will be attempting a fresh install of Sierra using all the same settings later tonight.
I am sure there is a way of doing this successfully - but, I wasn't.
Paul