I'm currently working on a mod which uses v1.8 of Update.esm. I'm considering updating the game to 1.9 - is there anything special I need to do in order to update the mod? Or just open the plugin and save?

Open your mod with update.esm 1.9, save it and then open it with TES5Edit. See if there is no conflict between your mod and the update.esm, correct things if necessary and save. At least, that's what I do and it seems good.