Saving guild war assignments (pressing the “Save Team Info” button) causes everyone’s defence teams to be updated; it causes a fresh “snapshot” of all guild war defences to be taken, which your opponent would then face. You don’t have to make any actual changes to participant composition or existing participants’s positions, as long as you trigger that save, from any fortress or the stronghold.
This means, all you have to do is go into any fortress or the stronghold, start assigning by pressing the assign button, and then you can immediately save everything by pressing the save button.
This can be tested by anyone who has permissions to edit a guild war composition (vice captains or guild masters), by editing their own defence team and then performing a variety of actions to see what causes their defence to update.
However, I am unsure if the following count as a “save” and cause a new snapshot to be taken:
- When a guild war’s “war phase” ends and the previous guild war composition is carried over to the next war’s “preparation phase”
- When a participant leaves or is kicked from the guild, causing them to be immediately removed from their tower/fortress/stronghold if it happened during a “preparation phase”, or if there’s still a war going on or if it’s the Saturday break, right when the next “preparation phase” starts.
If you know the answer to these specific cases (which I cannot easily test), please do leave a comment below.
(Could not ask the question while accepting my answer immediately on mobile, will do so after the two-day cooldown.)