Without the document it is difficult to test, but ...
Code:
Sub ChangeParasAfterHeading()
Dim para As Paragraph, nextPara As Paragraph
Dim oRng As Range
If Selection.Type = wdSelectionIP Then
MsgBox Prompt:="You have not selected any text!"
Exit Sub
End If
Set oRng = Selection.Range
With oRng
For Each para In oRng.Paragraphs
Select Case para.Style
Case Is = "Heading 1"
Set nextPara = para.Next
If Not nextPara Is Nothing Then
nextPara.Style = "Body1"
End If
Case Is = "Heading 2"
Set nextPara = para.Next
If Not nextPara Is Nothing Then
nextPara.Style = "Body2"
End If
Case Else
End Select
Next para
End With
End Sub