thatsa gd q, wat if someone wants to remove have of their gravel substrate and put some sand substrate to go along with your existing gravel substrate, can that be done w/out taking any fish out?
Probably would be fine. As long as you're leaving some of the gravel in there and not removing the filter from the tank while you do it, you've not going to lose the cycle. However, sand can be a big mess and make the water initially very cloudy. Rinse the sand very well first to get any debris out. Make sure the filter doesn't suck any of it up as you add it because it can get in the impeller (assuming you have a HOB filter) and make a terrible grinding noise! So turn off the filter while you do it, and lower the sand down to the bottom (maybe in a cup) and gently pour it.
IMO, sand and gravel don't look that great mixed. The sand mostly sinks to the bottom and gravel comes to the top.