about a year ago i changed my 46 gallon community to sand with absolutely no regrets - more natural looking and the fish enjoyed it more...
as for my 20 gallon planted, i started this tank off with sand (new tank) and noticed considerable growth over time....i decided to add a bag of laterite to the bottom 1/3 of sand and try planting again - made a huge difference and would really recommend it to you...
your best bet for the compacting issue, which has already been stated, is the addition of malaysian trumpet snails....usually if you look close at pet store tanks you can see these snails everywhere in the substrate (make sure they are not ramshorn or common pond snails that look really different and will eat plants)....
yes you can use a sponge filter no problem with the sand...the sand will only get sucked up if it is stirred up considerably
as for the amount of waste on top of the sand: i dont think it is that the debris is getting stirred into the sand - more like there are less places for the debris to go so more food is consumed when it reaches the bottom, and more debris is stirred up by the fish swimming moving it to the filter inputs......with gravel having so many crevices - it is the perfect place for excess food and waste to build up and never make it to the filter.....the signs of less debris on the sand is a very good thing!
overall, the switch should not require you to remove any fish...use something to scoop out as much gravel as possible, do a good vacuum and then use something to scoop sand in...you can use a plate on top of the sand to avoid stirring it up too much when add/readding water after tank cleanings.
and after reading your last post before submitting, you really should upgrade lighting to at least 2 watts per gallon if you would like to seriously plant it...sure you could use low light plants which wouldn't require co2 and most likely wouldn't need any extra fertilization, but you can get some decent lights and have a heavily planted tank - you'd probably have to get compact fluorescents which can be found for $50-100 online, opening up many more possiblities...feel free to PM me if you have any more questions about sand and live plants (sorry about the longwinded post haha)
