Changing Gravel In A Fishtank

I currently have small, smooth stones as gravel, but I'm considering switching to sand. Just wondering the best way in which this could be done, and would changing the gravel affect water quality due to the bacteria within the gravel I already have?

Thanks.

I typed a reply to this a couple of days ago but obviously forgot to click 'post' 


Basically I said the same as Colin. I have changed all three of my tanks from gravel to sand without problem. If the gravel is small enough to fit down the siphon tube, take the cylindrical part off and just use the tubing to remove the old gravel.
You can also remove the fish during the swap over. Save as much water as you can and keep the filter and decor in the saved water. Then just empty everything; this will get the muck from the gravel out. When you fill the tank back up with the saved water and new water it's just the same as a large water change.
Though if you usually do only small infrequent water changes, this might not be a good idea as your tank water will be different from your tap water.

Forgot that 

Wash the sand before you start on the tank!

Do it half way through a water change. Less water makes it a bit easier. And remove any decor that is not fixed into the substrate.
Use two nets or one net and a jar. Use one of the nets to chase the fish into the other net/jar.
If all the decor can be removed, use a barrier of some description to herd the fish into one corner and pen them there. Plastic craft mesh is ideal for this.

Some bacteria will grow on the substrate but most will be in the filter as conditions in there suit the bacteria best. The problem with changing half the substrate is that I don't know how you would keep the old gravel and new sand separated.
I changed all three of my tanks all at once with no problems (I did them one at a time, not all three on the same day!). If you reduce the amount you feed for a few days before and after the change over that will help. Under optimal conditions, the filter bacteria can double in something like 24 hours. Since you won't be removing anything like half the bacteria, you should be back up to full strength within a few hours. Don't forget that besides the bacteria inside the filter, there will be more on the glass walls and all the decor and plants in the tank. The substrate carries a small proportion of the total. The exception is undergravel filters where the gravel is the filter medium, but I assume you don't have that kind of filter