I have fairly large gravel in my tank, and I want bottom feeders eventually... but in order to get some, I've determined that I will need to change to a substrate with smaller particles. Currently, the substrate is just plain gravel from Home Depot, and I am concerned not only about how big and heavy it is, but it also doesn't have very round edges.
It's a 67g planted tank (see pictures below - the second is just a closer pic to show the gravel size). I probably wouldn't go for sand... I'm thinking stuff that has about a 2mm diameter. Is there a quick/easy way to go about doing this, or will it be as much of a pain in the butt as I've envisioned it to be? And I have heard that it's best to do it a little at a time, but how bad is it if I want to do it all in one shot? I'm picturing it being an all day event, and I don't want to have to do it more than once both for the sake of my sanity, and for the sake of not wanting to stress out the fish by putting them in buckets a billion times.
The way I've been envisioning it is to get some 5g buckets. After cutting power to the filter/heater/light, siphon water from the tank into each one. Put my 10 harlequin rasboras in one, 5 (quarter sized) angelfish in another, mollies and
Honey Gourami in another, and then the rocks, plants, and driftwood into another. How long could the fish stay housed in those temporary quarters without filtration and aeration?
then, drain the tank as low as it can go in order to get the tank lid off... (this is due to the fact that my lid and frame are all one piece, and my tank bows badly without the lid, so it's impossible to remove it while there's much water in the tank).
And then, switch the substrate... refill using water that's close to the temperature of the water in the temporary fish holding buckets (minus the amount that is in the buckets with the fish), and condition the water, replant/redecorate, and put the fish back in with the old tank water.
My question about "is there a quick/easy way to do this" is... can I get away with getting the fish out of the tank into buckets, and putting the smaller substrate on top of the existing substrate? (If your only response is "No, would you expect something involving fish to be easy? don't be so lazy" I won't be upset). :)
Although now that I've typed it all out, I'm thinking I could get away with not having remove the lid... I could feasibly just scoop it through the two openings in the lid (one is the part I open to feed them, and the other opening is where the light sits). hrm... then it wouldn't be so bad...
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What I would do if this was my tank (and we know it's not my tank!) is to purchase the substrate I wanted and wash the heck out of it. At the same time that I was ready to add the new substrate, I would also have a few more plants to add the tank. 
), cut off the "foot" and fill it with some of the current substrate and set it on your new substrate. It will act as a seeding source for the new substrate bed. After a few weeks toss the filled nylon sock. 
but i can buy a cheap pair for the sake of the fish. It's good to know i don'thave to remove the fish... I was thinking about how long it has taken me in the past to net a single rasbora... Let alone ten. Zippy little buggers! I just thought it'd be safer to remove the fish to avoid inflicting any fish concussions from falling rocks. I can just put it in by scoopfuls though rather than dumping it on them. My anxiety over this undertaking is slowly dropping. 
