Can I switch the ADC between an external reference and an internal reference?
No. The external reference pin, AREF, is connected directly to the ADC
reference input. When an internal reference is selected it would short
circuit to the external reference.
If you need multiple references you could use an external programmable
reference or an analog switch with multiple references or limit the
ADC range by choosing the highest reference voltage required.
Atmel. (2009). 8-bit AVR Microcontroller with 4/8/16/32K Bytes
In-System Programmable Flash.
, page 256. (Retrieved March
14, 2009, from doc8025.pdf