MySQL update a field with an incrementing variable
Simple query would be, just set a variable to some number you want. Then update the column you need by incrementing 1 from that number.
Ex:: Query to update all the rows where a column is null. it'll update each row id by incrementing 1
SET @a = 50000835 ;
UPDATE `civicrm_contact` SET external_identifier = @a:=@a+1
WHERE external_identifier IS NULL;
Are you looking for something like this?
UPDATE textile_events e,
(SELECT @n := 249) m
SET e.seq_no = @n := @n + 1
WHERE e.eid = 'headsup' AND e.paid = 'paid'
SQLFiddle