You don't need to toggle the design mode
Code:
Dim aCC As ContentControl
For Each aCC In ActiveDocument.ContentControls
Select Case aCC.Tag
Case "SecteurActivite", "Pays"
aCC.SetPlaceholderText , , "Choose an item."
Case "NomClient", "Site", "DureeProjet", _
"AnneeRealisation", "Description", "LibelleProjet", _
"SousTitre", "Benefices"
aCC.SetPlaceholderText , , "Click here to input text"
Case Else
aCC.SetPlaceholderText , , "no reason to display this one"
End Select
Next aCC
will do the job.