Ledger dimension vs Default dimension

 In short Ledger dimension it is Main account + default dimension.

So, if you need to replace any attribute value in a given Ledger dimension, do not forget to get a Default dimension first.

LedgerDimensionFacade::getDefaultDimensionFromLedgerDimension(ledgerjournalTrans.LedgerDimension);

Then you can use this old good way.

DimensionDefault newDim = DimensionHelper::setValueToDefaultDimension(hcmEmployment.DefaultDimension, DimensionAttribute::findByName(_dimensionName).RecId, _dimensionValue);
        

BTW, the opposite thing works like that:

LedgerDimensionFacade::serviceCreateLedgerDimension(ledgerDimensionMainAccount, inventTrans.defaultDimension);

Supporting method

public static DimensionDefault setValueToDefaultDimension(DimensionDefault _dimensionDefault, RefRecId _dimensionAttributeRecId, DimensionValue  _newDimensionValue)
    {
        DimensionAttributeValueSetStorage   dimStorage;
        DimensionDefault                    newDimensionDefault = _dimensionDefault;
        DimensionAttributeValue             dimensionAttributeValue;
        if (_dimensionAttributeRecId)
        {
            dimStorage = DimensionAttributeValueSetStorage::find(_dimensionDefault);
            if (_newDimensionValue)
            {
                dimensionAttributeValue = DimensionAttributeValue::findByDimensionAttributeAndValue(DimensionAttribute::find(_dimensionAttributeRecId), _newDimensionValue, false, true);
                dimStorage.addItem(dimensionAttributeValue);
            }
            else
            {
                dimStorage.removeDimensionAttribute(_dimensionAttributeRecId);
            }
            newDimensionDefault = dimStorage.save();
        }
        return newDimensionDefault;
    }

Bigger usage example

        // replace TransactionType to the one from parameters
        // first we need to get main account and default dimension from the originl transaction ledger dimension 
        mainAccountRecId = LedgerDimensionFacade::getMainAccountRecIdFromLedgerDimension(_ledgerJournalTransOrig.LedgerDimension);
        dimensionDefault = LedgerDimensionFacade::getDefaultDimensionFromLedgerDimension(_ledgerJournalTransOrig.LedgerDimension);
        // then replace this attribute with a new value
        dimensionDefault = myDimValueHelper::setDefaultDimensionValue(dimensionDefault, myDimensionConstants::TransactionType, LedgerParameters::find().myGLDimInvTransSale);
        // get default dimension for main account
        ledgerDimensionMainAccount = LedgerDefaultAccountHelper::getDefaultAccountFromMainAccountRecId(mainAccountRecId);
        // finally combine it with original main account to get a new ledger dimension
        ledgerJournalTrans.OffsetLedgerDimension  = LedgerDimensionFacade::serviceCreateLedgerDimension(ledgerDimensionMainAccount, dimensionDefault);
        ledgerJournalTrans.modifiedField(fieldNum(LedgerJournalTrans, OffsetLedgerDimension));

 https://alexvoy.blogspot.com/2015/10/how-to-lookup-and-set-new-value-for.html

Thanks Sasha Nazarov and Ievgen Miroshnikov and Denis Trunin