Migrate foreign key type from char to binary - ways to deal with the fallout?
Given the next example:

CREATE TABLE A 
(
    [ID] VARCHAR(10) NOT NULL,
    CONSTRAINT [PK_A] PRIMARY KEY ([ID])
);

CREATE TABLE B 
(
    [ID] INT PRIMARY KEY, 
    [A_ID] VARCHAR(10) NOT NULL,
    CONSTRAINT [FK_B] FOREIGN KEY ([A_ID]) REFERENCES A([ID])
);

CREATE TABLE C 
(
    [ID] INT PRIMARY KEY, 
    [A_ID] VARCHAR(10) NOT NULL,
    CONSTRAINT [FK_C] FOREIGN KEY ([A_ID]) REFERENCES A([ID])
);

INSERT INTO A VALUES ('001'), ('010'), ('100');
INSERT INTO B VALUES (1, '001'), (2, '001'), (3, '010');
INSERT INTO C VALUES (4, '010'), (5, '100'), (6, '100');


0- Backup, backup &amp; backup your database.

1- Due you are trying to convert varchar to varbinary and there isn't an explicit conversion you should add new columns to your tables. NOTE: You cannot make it non nullables.

ALTER TABLE A ADD [ID_VB] VARBINARY(10);
ALTER TABLE B ADD [A_ID_VB] VARBINARY(10);
ALTER TABLE C ADD [A_ID_VB] VARBINARY(10);


2- Copy current values to the new columns:

BEGIN TRANSACTION
  UPDATE A SET [ID_VB] = CAST([ID] AS VARBINARY(10));
  UPDATE B SET [A_ID_VB] = CAST([A_ID] AS VARBINARY(10));
  UPDATE C SET [A_ID_VB] = CAST([A_ID] AS VARBINARY(10));
COMMIT TRANSACTION


3- Drop current constraints:

ALTER TABLE B DROP CONSTRAINT [FK_B];
ALTER TABLE C DROP CONSTRAINT [FK_C];
ALTER TABLE A DROP CONSTRAINT [PK_A];


4- Once you have checked new values are correct, drop actual columns:

ALTER TABLE A DROP COLUMN [ID];
ALTER TABLE B DROP COLUMN [A_ID];
ALTER TABLE C DROP COLUMN [A_ID];


5- Rename new columns with old names:

EXEC sp_rename 'A.ID_VB', 'ID', 'COLUMN';
EXEC sp_rename 'B.A_ID_VB', 'A_ID', 'COLUMN';
EXEC sp_rename 'C.A_ID_VB', 'A_ID', 'COLUMN';


6- Make new columns not nullables:

ALTER TABLE A ALTER COLUMN [ID] VARBINARY(10) NOT NULL;
ALTER TABLE B ALTER COLUMN [A_ID] VARBINARY(10) NOT NULL;
ALTER TABLE C ALTER COLUMN [A_ID] VARBINARY(10) NOT NULL;


7- Add constraints again:

ALTER TABLE A ADD CONSTRAINT [PK_A] PRIMARY KEY ([ID]);
ALTER TABLE B ADD CONSTRAINT [FK_B] FOREIGN KEY ([A_ID]) REFERENCES A([ID]);
ALTER TABLE C ADD CONSTRAINT [FK_C] FOREIGN KEY ([A_ID]) REFERENCES A([ID]);


8- Check final result:

SELECT * FROM A;
SELECT * FROM B;
SELECT * FROM C;




| ID       |
| :------- |
| 0x303031 |
| 0x303130 |
| 0x313030 |

ID | A_ID    
-: | :-------
 1 | 0x303031
 2 | 0x303031
 3 | 0x303130

ID | A_ID    
-: | :-------
 4 | 0x303130
 5 | 0x313030
 6 | 0x313030



db&lt;>fiddle here