Hello,
It's not clear what you mean by leaving the light on. Presumably you perform the water change prior to the normal lights on time or at least at the beginning, correct?
If you do that then there is no extra few hours of light, it would be just the normal light period.
If you are performing the water change at the end of the photoperiod or at night when the lights are off then at the conclusion of the water change, leave the lights off. There would not be any benefit of turning the lights on during the normal lights off time.
Normally the WC is don just before lights on and yes there are two sources of extra CO2; the CO2 absorbed from the air when the plants are exposed as the water level drops, and some tap water contains CO2 - so this combination is an extra boos, and if the plants are cleaned and debris is removed from the surface of the leaves this also helps the plants to absorb CO2, so it win win if the WC is done at or prior to the beginning of the photoperiod. There is no point in raising the lighting level as this would tend to induce algal bloom, so just carry on with everything being normal.
Cheers,
