Light reflection from a charged surface

In case only charges are opposite, while masses remain the same, I would guess that you would only see the phase difference of $pi$ in the reflected wave in respect to the reflected wave with the original charges.

If masses are different, you can think of how mass is influencing the charge movement upon electric field, and I think you will find the answer.

It is unlikely that you would be able to see a change in reflectivity. Note that to change the electron density appreciably would require an unphysical amount of charge.