According to recent forum postings, some have received email & opted out via SMS.
I received email from the bank on 30May regarding Upgrade to HSBC Premier “we would like to upgrade your banking relationship….....The automatic upgrade will take effect after 29 Jun 2025……. If you prefer not to be upgraded, simply let us know by sending an SMS to …….. by 29 Jun 2025”. (Details for the SMS format also very clear)
I sent my SMS before 29Jun & received an immediate automatic SMS “Thank you for indicating your preference to not be upgraded……”