Sadly indeed charts are not converted when converted a xls file to xlsx/m. While we are working in a big overhaul of charts right now which will include rendering xlsx charts and adding them with code, chart conversion between xls and xlsx is still outside our scope.
The only solution here is to provide 2 templates: One in xls and the other in xlsm. When the user wants to save as xlsm you use the xlsm template, and if the user wants to save as xls, you use the xls template.