in my experence it is best to remove it all and then add the sand. just before you go adding it what sort of sand is it? sand isnt just sand! some sand is very carbonate rich which has the effect of increasing your pH. if you are using it for african rift lake chiclids then its perfect. if you are using it for bristlenose...disaster!
I use silica sand in my tanks becasue it is totally inert and will not affect the water in any way.
if you do plan on leaving the gravel in then you will need a lot of sand as the fish will quickly cause the sand to fall through the gaps in the gravel and it wont look as good. I would take the time and do it properly. took me about 3hrs when I switched from gravel to silica sand but it was well worth it!
_________________
Doug's Bristlenose eBook - The number one source of Bristlenose information and the most detailed breeding guide avaliable
3rd edition OUT NOW!!! LOADS OF ADDITION INFORMATION, PICS, DIAGRAMS AND BRISTLENOSE GOODNESS
GO TO www.keepingandbreedingbristlenose.com FOR YOUR COPY!!
Like my book - why not "Like" it on Facebook!
http://www.facebook.com/pages/Keeping-and-Breeding-Bristlenose-Catfish/142417842474993
Click here to vote for Bristlenose World