Would it be possible to use a portable power unit to plug into the usb to maintain a charge whilst changing the batteries so as not to loose the configuration?
Any portable mobile phone charger should work. You can try it out with a normal USB cable to a computer. You should be able to go from running just on batteries, to running on USB and batteries, to running on just USB without the AudioMoth stopping or losing its configuration.
I mean mobile phone charger
That is so cool....do I need to worry about what I buy re power input or would any mobile phone do ?
Yep, that should work fine.