You need to go into edit mode to change it. Open message > Actions (Move group) > Edit message.
if its something you'll do fairly often, you can use a macro - add it to a button and click it when you want to convert to plain text and reply.
This works with open or selected messages -
Sub ReplyPlain()
Dim objItem As MailItem
Dim oMail As MailItem
On Error Resume Next
Select Case TypeName(Application.ActiveWindow)
Case "Explorer"
Set objItem = Application.ActiveExplorer.Selection.Item(1)
Case "Inspector"
Set objItem = Application.ActiveInspector.currentItem
End Select
objItem.BodyFormat = olFormatPlain
Set oMail = objItem.Reply
oMail.Display
End Sub
From
Change format on replies