The Assign Field Values activity is limited in that it only takes a template name and not an ID, but it does allow you to select the template name using a token.  So you can set up a token value with the Template ID and then use an SDK Script activity to look up the Template name.

While this approach will allow for the template name to change, the Assign Field Values activity does not allow you to select fields using IDs or token values.  To assign both template and fields using IDs instead of the names, you would have to script the template and field assignments instead of using the built in activity.

        Protected Overrides Sub Execute()
            'Write your code here. The BoundEntryInfo property will access the entry, RASession will get the Repository Access session
            Dim sTName As String = Nothing
            Dim sPSetID As String = GetTokenValue("TemplateID")
            Dim iPSetID As Integer
            If Integer.TryParse(sPSetID, iPSetID) Then
                Try
                    Dim Tmp As TemplateInfo = Template.GetInfo(iPSetID, RASession)
                    sTName = Tmp.Name
                Catch ex As Exception
                    WorkflowApi.TrackError(ex.Message)
                End Try
            End If
            SetTokenValue("TName", sTName)
        End Sub