I couldn't find any way of doing it. Does anyone else know?
After some digging, this appears to be hardcoded. E.g., see "
<shift><control>I" when a new shortcut was added. There was also a shortcuts dialog added in July 2016 but it appears to show the shortcuts rather than let you edit them, and I'm not sure whether it made the cut into elementary's Epiphany.
So your best bet (without recompiling from source with your own fixes) is to use
sudo apt install autokey, which adds an easy interface for mapping a new shortcut to the existing one. You can also set the keyboard "phrase" to only trigger within Epiphany. Here is GNOME's list of shortcuts.