You need to sign the changing address form.

They need to verify your signature before can proceed the change.

It is for security reason.

Previously we have a fraud case involving broker/remiser which reported in newspaper whereby the broker that change the client CDS correspondence address, after that the broker sold the clients' CDS share, since the correspondence address is not the client address, the CDS statement never reach the client, so the client never knew his/her share was being sold. The brokers gone missing in action afterwards, after utilised the money from the sold shares to don't know where.

The CDS statement and investment house monthly statement are actually an important document to verify your trade.