SQL - merge update set conditional (each field)

Question

I have a question about sql merge

In the UPDATE section I set all columns which could have been changed. But I only want to set those who have really changed. Why ? Because several triggers will be fired upon this change and f.i. in the trigger I get an error because of condition UPDATE (PartGrpCode) is true. In the SQL-help it states UPDATE (in a trigger) does not check if it has changed.

        WHEN MATCHED THEN
    UPDATE 
    SET    [target].[PartCode] = [source].[PartCode]
    ,      [target].[QualityCode] = [source].[QualityCode]
    ,      [target].[PartGrpCode] = [source].[PartGrpCode]
    ,      [target].[VATCode] = [source].[VATCode]
    ,      [target].[DesignCode] = [source].[DesignCode]
    ,      [target].[FrozenCostPriceCurrCode] = [source].[FrozenCostPriceCurrCode]
    ,      [target].[PartMnem] = [source].[PartMnem]
    ,      [target].[PartType] = [source].[PartType]
    ,      [target].[ClassNr] = [source].[ClassNr]
    ,      [target].[StatisticNr] = [source].[StatisticNr]
    ,      [target].[Description] = [source].[Description]
    ,      [target].[Remark] = [source].[Remark]
    ,      [target].[Unit] = [source].[Unit]
    ,      [target].[Length] = [source].[Length]
    ,      [target].[Width] = [source].[Width]
    ,      [target].[Height] = [source].[Height]
    ,      [target].[Weight] = [source].[Weight]
    ,      [target].[SquareMeasure] = [source].[SquareMeasure]
    ,      [target].[Stand] = [source].[Stand]
    ,      [target].[BasicMat] = [source].[BasicMat]
    ,      [target].[Measure] = [source].[Measure]
    ,      [target].[InvtUnitOfAccount] = [source].[InvtUnitOfAccount]
    ,      [target].[InvtComputQty] = [source].[InvtComputQty]
    ,      [target].[UnitCode] = [source].[UnitCode]
    ,      [target].[PartObsInd] = [source].[PartObsInd]
    ,      [target].[InvtValueType] = [source].[InvtValueType]
    ,      [target].[TemplateInd] = [source].[TemplateInd]
    ,      [target].[LastUpdatedOn] = [source].[LastUpdatedOn]
    ,      [target].[LastUpdatedBy] = [source].[LastUpdatedBy]
           
           WHEN NOT MATCHED THEN

what i Need:

WHEN MATCHED THEN
UPDATE 
SET    [target].[PartCode] = [source].[PartCode]
,      [target].[QualityCode] = [source].[QualityCode]
,      case when [target].[PartGrpCode] <> [source].[PartGrpCode] 
            then [target].[PartGrpCode] = [source].[PartGrpCode] 
       end case   
,      case when [target].[VATCode] <> [source].[VATCode]
            then [target].[VATCode] = [source].[VATCode]
       end case

       WHEN NOT MATCHED THEN

Is this possible or with some other technique ?