EDIT

import eu.etaxonomy.cdm.api.conversation.ConversationHolderMock;

import eu.etaxonomy.cdm.model.taxon.ITaxonTreeNode;

import eu.etaxonomy.cdm.model.taxon.TaxonNode;

import eu.etaxonomy.taxeditor.navigation.navigator.TaxonNavigatorLabels;

import eu.etaxonomy.taxeditor.navigation.navigator.operation.RemotingChangeAcceptedTaxonToSynonymOperation;

import eu.etaxonomy.taxeditor.operation.RemotingCdmHandler;

        // check that only a single taxon tree node has been selected

        if(selection.size() > 1) {

            return new Status(IStatus.ERROR,

                    TaxonNavigatorLabels.SINGLE_TAXON_SELECTION_MESSAGE);

        }

        // check for no taxon tree node selected

        if(selection.size() == 0) {

            return new Status(IStatus.ERROR,

                    TaxonNavigatorLabels.NO_TAXON_SELECTION_MESSAGE);

        }

            oldTaxonNode = (ITaxonTreeNode)obj;

        } else {

            return new Status(IStatus.ERROR,

                    TaxonNavigatorLabels.SELECTED_OBJECT_NOT_TREE_NODE_MESSAGE);

        }

        // check that the source taxon node does not have children

        if(((TaxonNode)oldTaxonNode).getCountChildren() > 0) {

            return new Status(IStatus.ERROR,

                    TaxonNavigatorLabels.SOURCE_TAXON_HAS_CHILDREN_MESSAGE);

        }

        boolean editorClosed = NavigatorHandlerUtils.closeObsoleteEditor(event, (TaxonNode) oldTaxonNode);

        if(editorClosed != true) {

            return new Status(IStatus.ERROR,

                    TaxonNavigatorLabels.RELATED_EDITOR_NOT_CLOSED_MESSAGE);

        }

        excludeTaxa.add(((TaxonNode)oldTaxonNode).getTaxon().getUuid());

        TaxonNode newAcceptedTaxonNode = TaxonNodeSelectionDialog.select(HandlerUtil.getActiveShell(event),

                new ConversationHolderMock(),

                excludeTaxa,

                null,

                ((TaxonNode)oldTaxonNode).getClassification());