From the datasheet:
23.5.2 ADC Voltage Reference
"... The first ADC conversion result after switching reference voltage source may be inaccurate, and the user is advised to discard this result."
23.9.1 ADMUX – ADC Multiplexer Selection Register
"• Bit 7:6 – REFS[1:0]: Reference Selection Bits
These bits select the voltage reference for the ADC, as shown in Table 23-3. If these bits are changed during a conversion, the change will not go in effect until this conversion is complete (ADIF in ADCSRA is set)."