Many thanks to you both for your input.

This sub now works perfectly!

Sub FillForm()
    Dim oCtrl       As Control
    Dim oCC         As ContentControl
    Dim oRng        As Range
    Dim lngIndex    As Long
    Dim strTC       As String

    With m_oFrm
        
        For Each oCtrl In .Controls
            
            Select Case TypeName(oCtrl)
                Case "TextBox"
                    
                    If oCtrl.Name = "txtDescription" Then
                        Set oRng = ActiveDocument.SelectContentControlsByTag("Description").Item(1).Range
                        oRng.Text = StrConv(oCtrl.Text, vbProperCase)
                        oRng.Words.Last = UCase(oRng.Words.Last)
                        
                    ElseIf oCtrl.Name = "txtKeys" Then
                        Set oRng = ActiveDocument.SelectContentControlsByTag("Keys").Item(1).Range
                        If InStr(oCtrl.Text, "Yet to be determined.") = 1 Then
                            oRng.Text = StrConv(oCtrl.Text, vbLowerCase)
                            oRng.Words(1).Characters.First = UCase(oRng.Words(1).Characters.First)
                        Else
                            oRng.Text = StrConv(oCtrl.Text, vbProperCase)
                            oRng.Words.Last = UCase(oRng.Words.Last)
                        End If
                        
                    Else
                        ActiveDocument.SelectContentControlsByTag(Replace(oCtrl.Name, "txt", "")).Item(1).Range.Text = oCtrl.Text
                    End If
            End Select
        Next oCtrl
    End With
    
lbl_Exit:
    Exit Sub
End Sub