Retain previous data and update with new data

The tags suggest that this is in a trigger; therefore, one option is to use the DELETED and INSERTED tables. In other words, DELETED.SafetyStock is the previous, removed value; INSERTED.SafetyStock, the new. Hence, an UPDATE with a JOIN should suffice.

UPDATE tto
SET tto.SafetyStock = i.SafetyStock
  , tto.PrevSafetyStock = d.PrevSafetyStock
FROM database2.Warehouse tto
JOIN DELETED d ON d.PK = tto.PK /* e.g., Item ID */
JOIN INSERTED i ON i.PK = tto.PK /* should be same as above. */
;

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In my experience, the SET operations happen sequentially, but I showed example above as I do not believe you want to depend on the order of assignments. However, in theory, the following also should work.

UPDATE tto
SET tto.PrevSafetyStock = tto.SafetyStock
  , tto.SafetyStock = i.SafetyStock
FROM database2.Warehouse tto
JOIN INSERTED i ON i.PK = tto.PK /* e.g., Item ID */
;

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Or have another trigger on the database2.Warehouse table that maintains the PrevSafetyStock, but it would be prudent to reduce the amount of triggers (i.e., complexity) in the process.

I hope that helps!

Should this be a 'Instead of Insert' or 'After Update' ??

Not sure about the scripting

Thank you mwvisa1