3
The question is: how do you define your "former" members.
For a query, just run a Google search for something like "mysql change data in field on condition" and you'll find answers
like this, which recommends this SQL command to be executed in phpMyAdmin:
UPDATE my_table SET my_column='new value' WHERE something='some value';
which you'll need to adjust for XOOPS to something like:
UPDATE xxx_users SET notify_method=2 WHERE something='some value';
the "WHERE something='some value'" would be your definition of what the "former members" mean
