How do we get this is if we bought the game on GOG? I tried to use the download and a message said I need to install the game first even though it is installed via GOG.

EDIT: Nvm, it's there now.

During a deployment phase if you plan on upgrading AND using elite reinforcements on a unit then reinforce it first, THEN upgrade the unit. It's cheaper and will save a bunch of prestige over the course of a campaign.