(For save files made before June 16 2018)

Unfortunately, vanilla XCOM has no way of regenerating a soldier’s ability tree after its initial creation, meaning that it’s highly likely that you are still seeing Sustain as a Colonel-level perk instead of Heroes Never Die. This also means you won’t be able to upgrade Resurrect from its default 1-charge 8-HP form. If you’re willing to accept this or weren’t planning on using Resurrect in the first place, then you don’t have to do anything, but otherwise you have a few ways to get around this (besides just starting a new campaign):

Just to be safe, back up your save file before carrying this out.

Enable the game console by adding the line into XCOM 2’s launch options in Steam. If you’re using the alternative mod launcher, you would add this line in the launcher settings itself, at Settings > Edit.

Next, download shiremct’s [WOTC] Additional Soldier Console Commands mod. Once you’re in your game, go to the armory, view your valkyrie, and hit the tilde key to bring up the console. Type in which should give you the latest ability tree with Heroes Never Die. (For RPGO users, be warned that doing this will likely change the two squaddie abilities, as they are randomly chosen and are rerolled when regenerating the ability tree.)

A dirtier fix, but absolutely no risk of messing up your game.

Open up your C drive and go to the file path
You'll know you're in the right place if the folder has a file called MercyClassMod.XComMod. Once you're in there, go to the Config folder, then open XComGameData_MercyClassMod in a text editor of your choice. You should see something like this around the middle of the file:
Change RESURRECT_HP to a large number like 100 and RESURRECT_CHARGES to 2, then save the file. This will effectively give you the upgraded version of Resurrect right away when you unlock it, removing the need for Heroes Never Die. Just be warned that once you finish your campaign, you should return to this file and change these numbers back to their original forms.