From my experience, i washed the new substrate pretty good, left them in a bucket for 30min (or so), tested the water....(do your things for your specifically tank paramaters)
To start i removed about 35%-45% of water of my tank then I turned of my filter (no stress for the fish I guess) I get the new substrate and used a plastic cup to slowly drop them on some particulars areas and mixed as delicated as I could with a sharp plastic grabber. Some layers I left a new thick layer and propagated trimmed plants. I made possible to all snails and shrimps to run away from the cup before never see them again lol, but snails do survive.
However, my sword always had this segment of root system right at the bottom of its body, where the new leafs come from, and i still have to trim them and they are doing just fine. But I understand that it's not nice to see the roots from plants that looks nicer when growing from the ground by its own. I have shrimps and loaches living together so I leave it like that for them.
You can try plant weitghs, attach with aquarium plant glue, or simply hide with a rock or something.
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