Keep terms associated to nodes in migration

i think i found the fix but it bring me to ask :

actually

$this->addFieldMapping('taxonomy_vocabulary_1', '1');

could be

$this->addFieldMapping('field_taxo_vdl', '1');

so what happen to
taxonomy_vocabulary_1 because i get two similar term reference fields, only the taxonomy_vocabulary_1 is necessary .
the field_taxo_vdl field is convert from a content taxonomy field created into D6...than after the upgrade i get 2 similars fields..
could i delete one of them ? so keep only one mapping...
more generally is it better to transform the core taxonomy in D6, into content taxonomy fields before upgrading a website ?

thanks

ok, so can i keep the 2 mapping like this ?

$this->addFieldMapping('taxonomy_vocabulary_1', '1');
$this->addFieldMapping('field_taxo_vdl', '1');

the first to move vocabulary terms to the vocabulary_1 and the other to tag content.

actually in the ui (admin/content/migrate/groups/BL_migration/D2D_BriseLamesNodes_ContentSortie) for source i get :

• field_taxo_vdl,
• 1
• 2 ;

and in the destination :

• taxonomy_vocabulary_2 Taxo-tri-fonctionnel (taxonomy_term_reference)
• taxonomy_vocabulary_1 Type d'évènement (taxonomy_term_reference)
• field_taxo_vdl Type d'évènement (taxonomy_term_reference)

more generally,do i need to use the prepareRow function ?

new test with functional code (d7-grawitz.vmdev) and the migration replace existing term by a stub one, rollback migration doesn't get back my previous term..:(
my code:

  $this->addFieldMapping('taxonomy_vocabulary_1', '1')
      ->sourceMigration('Tri_fonctionnel') // This is the machine name of the tags migration.
         // This tells the migration to match on the term ID instead of the default, which is the term name.
      ->arguments(array('source_type' => 'tid'));
   $this->addFieldMapping('taxonomy_vocabulary_6', '6')
            ->sourceMigration('Actualites') // This is the machine name of the tags migration.
         // This tells the migration to match on the term ID instead of the default, which is the term name.
      ->arguments(array('source_type' => 'tid'));

Yes, i think i achieve this by changing the code like this :
this one create stub because it make reference of a migration already done when i upgraded the core.

$this->addFieldMapping('taxonomy_vocabulary_1', '1');
     ->sourceMigration('Tri_fonctionnel'); // This is the machine name of the tags migration.
         // This tells the migration to match on the term ID instead of the default, which is the term name.
      $this->addFieldMapping('taxonomy_vocabulary_1:source_type')
     ->defaultValue('tid');

So when i use this code, term is assigned to nodes as expected :

$this->addFieldMapping('taxonomy_vocabulary_1', '1');
      // This tells the migration to match on the term ID instead of the default, which is the term name.
      $this->addFieldMapping('taxonomy_vocabulary_1:source_type')
     ->defaultValue('tid');

but i get strange thing : the body content is display but when i go on edit mode, the field is empty..i'm using wysiwyg and CKE as editor.perhaps need another question ?