I am yet another victim of thinking my rpi 3 B+ was broken when it wouldn't boot up. It turns out that the 3 B+ is looking for some special boot instructions.
I currently have:
Ubuntu Mate 16.04 armhf - Boots on 3 NOT on 3 B+
Raspbian Stretch - Boots on 3 B+
Is it possible to copy files from the raspbian SD card to the Ubuntu SD card in order to make Ubuntu Mate boot on 3 B+? Note, I tried copying all files except for the kernel* files with no success.
No idea where this guy found / created a version of Mate that runs on 3 B+ but until a version exists from Ubuntu directly you can follow https://www.youtube.com/watch?v=J7xcd3I72Gw
Or download directly using this torrent.
It downloads as a zip and unzips as an iso. To install:
diskutil list diskutil umountDisk /dev/disk[get-number-from-list] sudo dd if=Path/to/UbuntuMate.iso of=/dev/disk[get-number-from-list] bs=1024m
Plugged sd card into the rpi and violá. Display, mouse, keyboard, bluetooth, wifi, it all works.
Regarding the original question I would still know how to perform the boot upgrade myself, ultimately so I can do it for 14.04 trusty.
The person who made the ubuntu 16 armhf iso said he just copied files over from raspbian. So far my attempts at this have failed but once I figured it out I'll post back with an answer.