Graham's method may work but it will effect the typed text as well. If you want the PHT to be bold and the typed text normal then you could set the PHT to saved building block entry of the PHT with block font) e.g.,
Code:
If A Then
ActiveDocument.ContentControls(1).SetPlaceholderText ThisDocument.AttachedTemplate.BuildingBlockEntries("PHT1")
Else
ActiveDocument.ContentControls(1).SetPlaceholderText ThisDocument.AttachedTemplate.BuildingBlockEntries("PHT2")
End If