I just downloaded the upgrade and flashed it with fastboot. Everything seems to work fine and nothing is lost if you choose to upgrade with fastboot. If you have TWRP installed it will be removed, but you can boot this image here:
https://sourceforge.net/projects/re...wrp-3.7.0_12-v6.5_A13-munch-skkk.img/download
And after you boot the image, you can choose in TWRP to install it. Don't flash it directly, just boot it with: fastboot boot twrp.img.