A simple way to update your Design Days would be to
- create an OpenStudio file with the data from appropriate ddy-file loaded.
- Then export the osm-file e.g. to file aux.idf
- Then copy manually the design days in IDF Editor from aux.idf to your idf file.
- Run test simulation and check whether target temperatures etc. remain within desired bounds.
- If OK, you will probably not need 12 sizing periods.
