I'm running kernel 3.4 /w u-boot v2014.07 on NanoPi2.

I would like to upgrade kernel to 4.4 and apparently I need to upgrade U-boot too.

I have compiled U-boot v2016.01 but I'm stucked /w this statement on the wiki:

"Note: you cannot use mixed U-Boot files. For example you cannot use fastboot to update an existing U-Boot V2014.07 and you cannot use bootloader.img to replace an existing u-boot.bin"

What to do next ? Can't I just do this ?
fastboot flash bootloader bootloader.img