A PR is a PullRequest on github, is a petition to merge new code into the main codebase.
here are some comments about this: Lede on Pogoplug Pro, Bad Magic Number
I think that is not necessary to upgrade uboot, just change uboot config to boot properly, it seems that there is a way to config uboot to point to a newer uboot and config that too... (seen on that forum thread), My notes worked perfectly for OpenWrt 15.05.1, and the @bobafetthotmail worked for him using LEDE 17.01.4, I've not tried yet because of lack of complete instructions to test, and I had no time these days for compile it by myself neither, also, I never compile, the first time I've tried it takes so long, so I always build my images using precompiled packages. So I think I will wait until a proper step by step guide on how to upgrade from OpenWrt 15 to Lede 17, or for a new OpenWrt 18...