Matt beat me to it!
Have you used root tabs for the plants rooted in the gravel? The anubias should be attached to decor so that needs just a liquid fertiliser but most plants in the substrate need root tabs. And I am aware that after a year or two some plant substrates become inert like your current sand and root tabs then become necessary.
I don't know what fish you have but if there are bottom dwelling fish, remember to check if your new substrate is smooth enough for them.
I would not like to change substrate with fish in the tank. I changed from gravel to sand several years ago, but I did it at the same time as I moved the tank across the room so I had to remove the fish and empty the tank to move it.
The amount of muck that came out of the gravel had to be seen to be believed. Sand may not have quite the same amount but there will still be a lot, and sand also runs the risk of gas pockets. I have read recently of someone who changed their sand substrate with fish in the tank and a lot of the fish died.
If you are quick with the swap, the fish will be OK in a bucket - covered with something, even a cloth - for a couple of hours. Plan everything first. Get the substrate washed ready. Then siphon water into the bucket, catch the fish and put them and the decor in the bucket. Then siphon the sand out, and you'll probably need to remove all the water as well as it'll probably be very messy by the time the sand is all removed. Once the new substrate and plants are in the tank, fill it with new water and put the decor and fish back in.
I will let someone else describe how to put the plants in the new substrate as all my plants are either floating or attached to decor.
When you say you've hoovered up sand, have you not washed it and put it back in the tank? That's what I do.