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

D365FO data entities with financial dimension requires special field names

 While creating data entities using financial dimensions, do not forget that those are backed in background by special classes via SysDataEntityPersister class

Those latter help resolving selected values (similar to what we see in segmented entry in forms). 

Each class is meant to serve a particular EDT, for example

/// <summary>
/// The <c>LedgerAccountDimensionDataEntityResolver</c> class resolves data entity ledger account values.
/// </summary>
[
System.ComponentModel.Composition.ExportMetadataAttribute("DimensionSFKType", identifierstr(LedgerDimensionAccount)),
System.ComponentModel.Composition.ExportAttribute("Microsoft.Dynamics.AX.DimensionDataEntitySFKFieldResolver")
]
public class LedgerAccountDimensionDataEntityResolver extends DimensionDataEntitySFKFieldResolver
{
    Name accountStructure;

...

In order to let it work correctly, some fields in your data entity must have particular names.

public class DimensionDataEntityConstants
{
    public static const str HasDimensionDataSourcesCustomPropertyName = '__HasDimensionDataSources__';
    public static const str ProviderCacheName = 'DimensionDataEntitySFKProviderCacheName';
    public static const str AccountStructureSuffix = 'AccountStructure';
    public static const str DisplayValueSuffix = 'DisplayValue';
    public static const str DisplayValueTypeName = 'DimensionDisplayValue';
}