Or you can change the configuration file. The only thing that seems to be changed when the PDF/A option is toggled on is a number in the .libreoffice/3/user/registrymodifications.xcu file. The entry <item oor:path="/org.openoffice.Office.Common/Filter/PDF/Export"><prop oor:name="SelectPdfVersion" oor:op="fuse"><value>0</value></prop></item> has the value changed from 0 to 1.