If the *only* thing you changed or added was a username/password, you do NOT have re-export/import the .ovpn file to the client. The username/password is never provided in that config file anyway. All you will ever see is the following directive.
Because there's no file being passed as an argument w/ this directive (which if it was, would normally contain the username/password on separate lines), the OpenVPN client will prompt you for the username/password at runtime, when it's needed.
And if you think about it, how would the router even know which username/password it should provide during the export/import? If that could be communicated somehow, I suppose it's possible the export/import process could also configure and export a username/password file (e.g., userpass.txt) and pass it as an argument w/ the auth-user-pass directive.
Code:
auth-user-pass userpass.txt
But as far as I can see, at present it doesn't do that.
Just guessing, but what might have happened is that you also changed the "Username / Password Auth. Only" option from No to Yes, or vice versa. THAT would require a re-export/import of the client's config file since that determines if the client's cert and private key are also required to authenticate the connection.