Thanks for the lovely comments
Jackie it's quite simple to do but can be time consuming. Unfortunately you will have to remove all the plants / fish / decor. The best way to do it is to get a large enough container to hold all the fish for a few hours,fill with tank water, put your heater in the container to keep the water warm. I didn't do this as I had to split my fish between 4 containers but left them infront of a warm radiator and the temp didn't appear to drop at all. If you have an internal filter you could run that in there as well or if you'd rather not then have a seperate bucket of tank water with the filter in, as long as the bacteria remains wet, it should be fine for a few hours. Ideally you want to save as much tank water as possible 50% + would be great(because then in theory your only doing a 50% or less water change).
Once you have everything out remove the rest of the water and remove the gravel, I found a new unused dustpan / brush set the quickest method because i could literllay sweep up the gravel and chuck it away. You may want to take this opportunity to give the tank a really good clean as well. Once the tank is empty you can add the sand, oh I forgot to add if you can wash the sand before you start it all, failing that wash it at this point a bucket at a time before adding(if you don't wash it you will end up with a cloudy mess for days). Once all the sand is added place something on top of it such as a plate and then a bowl / cup on top of the plate that you can pour the water in, I used a new plastic tray I had, because once the water started to fill up the tray floated so posed less risk of disturbing the sand.Whilst you are filling turn off the heater in the conatiner of fish(if you have a spare heater this isn't necassary)
Once you've filled it to about halfway(presuming you removed 50% of the tank water initially) then start adding the old tank water, add as much as you can without taking too much away so that the fish can barely swim(this will stress them out). 75% full should be about right, so you'd be removing half the old tank water from the container. Your heater should have cooled down by now, so you can transfer it over to the tank(or put the spare in whichever your doing) turn it on and wait for the water to get to the right temperature, it shouldn't take too long if you've manually tried to match the temperature of the water you were adding(rather then filling up with just cold water). Once your satisfied that the water is roughyl the same temperature you can add all the plants / decor and then finally the fish. Add the remaining tank water from the container, plus your internal filter(if that's what you have) and voila, done.
It took me about 3 or 4 hours to do the 66G above, which included washing the sand, netting the fish, giving the tank a good scrub, rearranging the decor several times
It took me about an hour to do the 15G which again included netting the fish, washing the sand etc.
Hope that helps and isn't too confusing or not clear, if you have any questions then feel free to ask
Oh also keep an eye on the water quality, some people experience a mini cycle when doing this as removing the gravel will remove a small amount of beneficial bacteria, I personally didn't have a mini cycle but just thought i'd warn you so you can keep on top of it.