This is how I did: I dropped the original column, and added a new column for auto increment first and set auto_increment to 1 as well.
mysql> alter table employees drop column sid, add column sid int(11) unsigned not null auto_increment first, drop primary key, add primary key (sid), auto_increment=1;
Query OK, 0 rows affected (14.42 sec)
Records: 0 Duplicates: 0 Warnings: 0
Please note that if you have constraints on this column, you should drop it before executing above statement.