If you have your heart set on changing your gravel to something else (I'd suggest a nice coloured micro gravel) it's not too horrible to do. I changed my 37 litre tank mid-year and had no problems with it at all (after getting some advice here). I did the following:
You will need a bucket (cheap bucket that you will never use for anything else, black is a good colour as it will persuade the fish it's night and they'll not stress as much) - I will assume you have a 10 litre bucket (can be more, but you get the jist).
[*]Prepare new substrate (wash etc) and place into a container of some kind
[*]Prepare enough new tank water to fill up the tank, less 10 litres
[*]Switch off filter & heater
[*]Fill up the bucket with tank water
[*]Transplant filter into bucket, switch on filter
[*]Catch all live stock and put into bucket
[*]Drain the tank into at least two containers
[*]Remove plants, rinse in one waste container and then place into another
[*]Do the same as above for any hardscape
[*]Scoop out the old substrate into a tub, being careful not to scratch the glass (this can take quite a while, a dustpan with rubber lip works really well)
[*]If you need to, clean the sides of the tank - don't do the bottom as it'll be covered in bacteria and hidden by the new substrate
[*]Pour any "mulm" (brown water) from the bottom of the old substrate into the container of new substrate, this will put some bacteria into the new substrate
[*]Place new substrate into the tank, back should be higher than the front to create the illusion of depth
[*]Plant plants - feel free to part fill the tank up with some of the new water
[*]Pour in all of the new water & arrange other stuff in there (bogwood etc)
[*]* If you've not warmed your new water, put in your heater and run that until the light goes off
[*]Switch filter off and clean in one of the tubs of waste water
[*]Put the filter into the tank, placing it below the water line and switch on
[*]When the water is clear (max of 30 minutes) switch the filter off and start transplanting the fish back
[*]Top up the aquarium with the 10 litres in the bucket, you should have a litre too much (the volume of substrate and other bits) - this is good as it means you don't have to pour in any fish waste that has settled at the bottom
[*]Arrange the filter and heater into their correct locations and switch on
[*]Leave the light off for the night to let everything settle to the new arrangement
[*]Time for a coffee
Took me around 2 hours (excluding the first two steps) to do mine and I think the most the filter was switched off was 10 minutes when I cleaned it. I had no ammonia spikes or other issues.
