ISet features = feature.Split(splitLine);

//let's say the original feature had surrogate id 1,
//so both split features have surrogate id 1 at the moment

//let the next available surrogate id be 6
IFeature splitFeature = (IFeature)(features.Next());
splitFeature.set_Value(surrogateIdOrdinal, 6);
splitFeature.Store();
splitFeature = (IFeature)(features.Next());
splitFeature.set_Value(surrogateIdOrdinal, 7);
splitFeature.Store();

IQueryFilter filter = new QueryFilterClass();
filter.WhereClause = surrogateIdField + " = " + surrogateId;
return fClass.FeatureCount(filter) > 0;

IDataChangesEx changes = workspace.get_EditDataChanges(esriEditDataChangesType.esriEditDataChangesWithinSession);
IDifferenceCursorEx = changes.ExtractEx("fclass", esriDifferenceType.esriDifferenceTypeInsert);
IRow sourceRow, differenceRow;
ILongArray differenceIndices;
int oid;
differenceCursor.Next(out oid, out sourceRow, our differenceRow, out differenceIndices);
//sourceRow has OID 100 and surrogate id 6

public bool existsBySubsId(int subsId, out bool foundInAllRows)
{
    foundInAllRows = false;
    IFeatureClass fClass = featureWorkspace.OpenFeatureClass("fclass");
    int subsIdOrd = fClass.FindField("SUBSID");
    IFeatureCursor cursor = fClass.Search(null, true);
    IFeature feat;
    while ((feat = cursor.NextFeature()) != null)
    {
        int foundSubsId = (int)feat.get_Value(subsIdOrd);
        if(foundSubsId == subsId)
        {
            foundInAllRows = true;
            break;
        }
    }

    IQueryFilter filter = new QueryFilterClass();
    queryFilter.WhereClause = "SUBSID = " + subsId;
    return fClass.FeatureCount(filter) > 0;
}

//feature has SUBSID = 5
public ISet SplitFeature(IFeature feature, IPolyLine splitLine)
{
    ISet splitFeatures;
    bool subsIdExists, subsIdFoundInAllRows;
    subsIdExists = existsBySubsId(5, out subsIdFoundInAllRows);
    //both subsIdExists and subsIdFoundInAllRows are true
    splitFeatures = feature.Split(splitLine);
    subsIdExists = existsBySubsId(5, out subsIdFoundInAllRows);
    //subsIdExists is false, subsIdFoundInAllRows is true
    return splitFeatures;
}

IQueryFilter filter = new QueryFilterClass();
filter.WhereClause = "SUBSID = 1";
IFeatureCursor cursor = featureClass.Search(filter, true);
IFeature feat = cursor.NextFeature();
//feat is not null even though there are no features with SUBSID == 1
int subsId = (int)feat.get_Value(feat.Fields.FindField("SUBSID"));
//subsId is 2