MySQL 8.0

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CREATE TABLE example (
  Revision VARCHAR(10),
  Hidden BIT
);
INSERT INTO example VALUES
  ('X', 1),
  ('X', 0),
  ('X', 0);

Records: 3  Duplicates: 0  Warnings: 0


SET @count = 64; -- so the first increment is 65 (starting on A).

UPDATE example t
  SET t.Revision = CHAR(@count:=@count+1)
WHERE t.hidden = 0;

Rows matched: 2  Changed: 2  Warnings: 1


SELECT * 
FROM example;


Revision
Hidden


X
1

A
0

B
0


DELETE FROM example;

INSERT INTO example VALUES
  ('X', 1),
  ('X', 0),
  ('X', 0);

Records: 3  Duplicates: 0  Warnings: 0


UPDATE example t, (SELECT @count:=64) t2
SET t.Revision = CHAR(@count:=@count+1)
WHERE t.Hidden = 0;

Rows matched: 2  Changed: 2  Warnings: 2


SELECT * 
FROM example;


Revision
Hidden


X
1

A
0

B
0