I have a dimmer switch that works the over head light. We are going to put a ceiling fan up where the light is, I wanted to know if we would have to replace the dimmer switch.

No. You will still be able to use the dimmer switch to dime your light on the fan. If it is just a single switch you will have to turn the fan on at the chain and the dimmer switch for the light.

It really depends on exactly what kind of dimmer switch you have. If it is a slide switch or knob that turns it must be replaced. There are switches that work with fans but they are stepped switches, usually 3-4 settings but not a true dimmer.

Short answer is YES, replace it. When you take the light down, look and see what wires are in the ceiling and we can re-advise you.

Ceiling fan remote controls - help and FAQ- Ceiling Fans N More (http://www.ceiling-fans-n-more.com/ceiling-fan-controls.php)