2 queries:

SQL Query
alter table ubbt_USER_PROFILE
change USER_ACCEPT_ADMIN_EMAILS USER_ACCEPT_ADMIN_EMAILS varchar(3) not null default 'yes'

SQL Query
update ubbt_USER_PROFILE
set USER_ACCEPT_ADMIN_EMAILS='yes'

Just a note on the last one. Unless you have something in your board rules, users that have elected to not receive admin emails might accuse you of spamming when they are forced to accept them.