Using data merge or data modify to change the BlockState.Name or BlockState.Properties tag for a falling_block entity will not change how it looks, but it will still settle as the block it was modified to.
To replicate:

summon falling_block ~ ~ ~

data modify entity @e[type=falling_block,limit=1,sort=nearest] {BlockState:{Name:"minecraft:stone"}}

It will continue to look like sand, but once it lands it will place a stone block