Nevets,

The following is not pretty - needs error handling around things like
ensuring the dropdown is a dropdown and the bookmark exists - but it works.
Also, I'd probably use an AutoText entry with the bookmark included so as to
avoid using the Selection object, but at least it will give you someplace to
start.

Sub InsertNextPara()
Dim DDSelection As Integer
DDSelection = ActiveDocument.FormFields("Dropdown1").DropDown.Value
   Select Case DDSelection
   Case 2
       ActiveDocument.Unprotect
       ActiveDocument.Bookmarks("NextPara").Select
       With Selection
           .Expand wdParagraph
           .End = .End - 1
           .Range.Text = "This is the text for XXXX."
       End With
       ActiveDocument.Protect wdAllowOnlyFormFields, True
   Case 3
       ActiveDocument.Unprotect
       ActiveDocument.Bookmarks("NextPara").Select
       With Selection
           .Expand wdParagraph
           .End = .End - 1
           .Range.Text = "This is the text for YYYY."
       End With
       ActiveDocument.Protect wdAllowOnlyFormFields, True
   Case 4
       ActiveDocument.Unprotect
       ActiveDocument.Bookmarks("NextPara").Select
       With Selection
           .Expand wdParagraph
           .End = .End - 1
           .Range.Text = "This is the text for ZZZZ."
       End With
       ActiveDocument.Protect wdAllowOnlyFormFields, True
   Case Else
       MsgBox "You must select a value.", vbCritical, "Dropdown Error"
       ActiveDocument.FormFields("Dropdown1").Select
   End Select
End Sub

I'm assuming you know how to set up the formfield to run the macro on exit.

Only problem: I can't seem to get the focus to remain with the dropdown
after clearing the message box, even if I try selecting it twice.