[FIX] CAS login not merging users with local accounts (#18238)

app/cas/server/cas_server.js

@@ -179,11 +179,12 @@ Accounts.registerLoginHandler(function(options) {
 	// First, look for a user that has logged in from CAS with this username before
 	let user = Meteor.users.findOne({ 'services.cas.external_id': result.username });
 	if (!user) {
-		// If that user was not found, check if there's any CAS user that is currently using that username on Rocket.Chat
+		// If that user was not found, check if there's any Rocket.Chat user with that username
 		// With this, CAS login will continue to work if the user is renamed on both sides and also if the user is renamed only on Rocket.Chat.
+		// It'll also allow non-CAS users to switch to CAS based login
 		if (trustUsername) {
 			const username = new RegExp(`^${ result.username }$`, 'i');
-			user = Meteor.users.findOne({ 'services.cas.external_id': { $exists: true }, username });
+			user = Meteor.users.findOne({ username });
 			if (user) {
 				// Update the user's external_id to reflect this new username.
 				Meteor.users.update(user, { $set: { 'services.cas.external_id': result.username } });