Actually, changing between two extremes could be damaging.
One of the most damaging cycles for any chip that runs hot is the continued heating and cooling when being switched on and off.
For example, in this case you want to overclock the card while playing games (making it run in effect at its max limit, thus max temp.) then when not playing games, reduce its speed to standard (no work load, runs at low temp.). This will cause the same type of heating and cooling.
Most people just run their cards at an overclocked speed and dont worry about it (including me) and while prolonged overclocking (and over-heating) can be bad, no-one is sure yet what effect constant heating and cooling have on chip life cycles.