Change it once, then save that file as a template file. Place the new template in a logical location (Graphisoft's standard install creates a Template folder). Then when you open ArchiCAD and start a new project, select the "Use a Template" and then browse to the template previously created (the next time this will already be selected) and you will have a new file with your units set up as you like them.
I also strongly suggest you make use of templates to make other settings that you find yourself using over and over such as views in the view map, pen sets, default tool settings, etc.
I hope this helps.
Erich
AC 19 6006 & AC 20
Mac OS 10.11.5
15" Retina MacBook Pro 2.6
27" iMac Retina 5K