Howto reboot to DFU factory image

[mmar]

Hi ,
is possible program this idea. In upgrade APP mode code user start DFU download.
Code in core reboots and start DFU mode, but loader use valid code in upgrade part. I need change this .

I know , that if update part is erased or corrupted loader automatic load factory , but can this be forced other way?
I plan use factory as slim loader (low flash usage as full app) and place full app to update part.