Please allow us to update the game without active subscription

[Felis]

Unless he has existing character, which the trial account won't see. Still, this is a good idea if you can install the trial to a different folder, then after it's downloaded and updated, copy the updated files into the active game folder. (Assuming you can find out which files to copy.)
One the updated game with original characters is up and running, close and uninstall the trial.

different accounts don't need different clients.
Create a trial account -> login to the trial account -> the launcher start to download the missing patches -> relog to the normal account after paying the sub -> all patches already downloaded