Does yesterday's filesystem update call for a new installation image?
I don't understand... are you talking about archiso? If so then you should know that it was intentional that archiso was created and released before the /usr merge. This was so that it would be a reliably stable media that wouldn't be potentially plauged by issues with the move if some had arisen. It doesn't matter if you use this months iso, last months, or several months ago. It will always install the most recent packages since the only option is to do a net install.