Unfortunately I just cant process all the mods that people have in mind. The board is designed to take a 3.7V lipo.
I cant really predict how things will fail and how to fix, undo, properly mod, etc, when higher VINs are connected when 6V max is expected on on the VIN pin.
There is the schematic for reference and any mods and hacks, cut traces and things like that are not endorsed or supported. You might render the board useless.
As a rule of thumb, it is very uncommon and not recommended to supply low power electronics from anything over 5-6V or so. 9V batteries are small capacity and will waste lots of that energy when fed into an LDO, 12v, 15v, 24v situations should be regulated with an external efficient DC/DC regulator, rather than try to feed that into a small linear regulator that is most efficient at around 4V anyway.