Gravel change?

[Bill Tune]

I currently have some relatively course gravel in my 280litre tank. This seems OK for plants like swords & java fern, but I also want to plant some smaller varieties which do better with finer gravel.
Can I just put the finer gravel ontop of the existing, or should I pull all the gravel out & replace with new stuff?

[ILLUSN]

if you just add finer gravel to your tank, it will eventually work its way down to where the roots want it.