AFG3000: Output Load selection+ does it change the AFG output impedance?

No, it does not, the output impedance of the AFG3000 series is always 50 ohms.

The three selections allow the output Amplitude to be calculated correctly based on the DUT load. i.e. (setting at 50 ohms) With a 50 ohm source and a Hi-Z load the output level on the screen will show as 1 Vpp but the measured output into a Hi-Z will be 2 Vpp. If you were to change the selection to Hi-Z then the screen would read 2 Vpp the same as the measured value. Like wise with a 50 ohm source and a 25 ohm DUT load the screen will show 1Vpp but the measured value at the DUT will be 0.666 mV.  Here your change the Load to be 25 ohms then the screen will read 0.666 mV.