I have been a long time OpenOffice user now. It is not possible to change the name of the template in an Impress document. According to the OO Impress Guide there has even been a bug that disabled automatic update of changed template styles.
Fortunately the guide shows a macro to fix this :
Sub FixDocument TemplateName = ThisComponent.DocumentInfo.Template if TemplateName <> "" then ThisComponent.DocumentInfo.Template = "name of the template" end if End Sub