Updating Cost Centre on Costing of Position using HDL

Summary:

We are trying to update the Cost Centre on the Costing on Position using HDL.

Each time we try it overwrites the original effective date of 01/01/1951
For example we have a cost centre of R0000 and for example a position code A1122 and the effective date is 01/01/1951
We are trying to "update" the cost centre as of 01/09 (Sept)/2024 with a new cost centre code of R0042 with the same position A1122
Using this HDL:

METADATA|CostAllocationAccountV3|SourceType|PositionCode|BusinessUnitName|LegislativeDataGroupName|EffectiveDate|SourceSubType|Proportion|SubTypeSequence|Segment2
MERGE|CostAllocationAccountV3|POS|A9969|The ######### ##### Ltd|GB Legislative Data Group|2024/09/01|COST|1|1|R0042

But as i have advised its overwriting it and not Updating it via this HDL, any advise please as the object source doesnt have headers for EffectiveStartDate or EffectiveEndDate just has EffectiveDate