Question

Can I switch the ADC between an external reference and an internal reference?

Answer

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.

References

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)