If a user accidentally deletes their Administrator account, they will need to register again and run the appropriate query for their SMF version. If a user loses their Administrator membergroup and privileges due to a mistake or conversion, they will need to run the appropriate query for your SMF version, but you do NOT have to re-register.

The below queries will add you back as an Administrator on an SMF forum.

SMF 1.1

Run this query in phpMyAdmin:

UPDATE smf_members SET ID_GROUP = '1' WHERE memberName = 'adminsusername'

SMF 2.0

Run this query in phpMyAdmin phpMyAdmin:

UPDATE smf_members SET id_group = '1' WHERE member_name = 'adminsusername'

Alternatively, SMF 2.0 allows these people to re-attribute posts through the Forum Maintenance section of their Administration Center (Administration Center > Forum Maintenance > Reattribute User Posts). They just need to simply fill in the form using the e-mail or username of the old user, and the username for the user who is to have those posts. This only works for guest posts (remaining posts from deleted members are considered guest posts).