It probably would have been easier to remove the fish and do it the way you mentioned Hippy but I tried to decide which would be less stressful to the fish as I don't like to remove fish from a tank unless absolutely necessary.
I just took about 2 handfuls of old gravel out at a time and made sure there was a big enough gap between the old/new each time. It was to change the colour of the gravel, so luckily I had two different colours for a while which made it easier if some did get mixed up. This way I also thought that the new gravel would have a chance to establish more bacteria if some of the old was still in there.
I made sure to place the new gravel right on the bottom without letting any fall or make to much noise and luckily the fish didn't mind at all
when I say a handful it was probably more like a desert spoon full in my hand each time.
I'm sure your way is best Hippy
and it would be quicker than the way I did it for sure. If you had a large tank and did it my way it probably wouldn't work as it would take far too long.