Don't want to write this by hand? GodCraft sets minecraft:replaceable for you with a switch.Get GodCraftOverview
minecraft:replaceable is a block component: it goes in the components object of a custom block. Lets another block be placed right where this one is, replacing it, the way placing a block on tall grass or snow layers swaps them out.
Example
Inside the components object of BP/blocks/<name>.json (values are the defaults or placeholders):
BP/blocks/<name>.json
{
"minecraft:replaceable": {}
}