SQL Server 2019

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CREATE TABLE dbo.Leavers(Actual_Termination_Date varchar(MAX)); --Why do you need 2GB to store a date?

INSERT INTO dbo.Leavers
VALUES (NULL),
       ('13/04/2017'),
       ('14/04/2017'),
       ('17/04/2017'),
       ('19/04/2017'),
       ('21/04/2017'),
       ('23/04/2017'),
       ('24/04/2017'),
       ('26/04/2017'),
       ('28/04/2017'),
       ('29/03/2017'),
       ('29/04/2017'),
       ('30/04/2017'),
       ('31/03/2017'),
       ('42795'),
       ('42797'),
       ('42813'),
       ('42817'),
       ('42820'),
       ('42825');
20 rows affected

UPDATE dbo.Leavers
SET Actual_Termination_Date = CONVERT(varchar(8), ISNULL(TRY_CONVERT(date, Actual_Termination_Date, 103), DATEADD(DAY, TRY_CONVERT(int, Actual_Termination_Date),'18991230')), 112);
20 rows affected

SELECT *
FROM dbo.Leavers;

Actual_Termination_Date

null

20170413

20170414

20170417

20170419

20170421

20170423

20170424

20170426

20170428

20170329

20170429

20170430

20170331

20170301

20170303

20170319

20170323

20170326

20170331

ALTER TABLE dbo.Leavers ALTER COLUMN [Actual_Termination_Date] date;
SELECT *
FROM dbo.Leavers;

Actual_Termination_Date

null

2017-04-13

2017-04-14

2017-04-17

2017-04-19

2017-04-21

2017-04-23

2017-04-24

2017-04-26

2017-04-28

2017-03-29

2017-04-29

2017-04-30

2017-03-31

2017-03-01

2017-03-03

2017-03-19

2017-03-23

2017-03-26

2017-03-31

DROP TABLE dbo.Leavers;