Compatible: You start a Pumpkin server, vanilla clients can join and play.
Drop-in: You run a server for some time. You decide to switch the software by replacing the executable. Everything works as before.
Drop-in: You run a server for some time. You decide to switch the software by replacing the executable. Everything works as before.
This is especially true with complicated vanilla Minecraft setups and red stone machines (Java Minecraft red stone has "bugs" that "shouldn't be there" but cannot be removed now since so much depends on it).
By the time you are 'compatible' then you have implemented everything needed to also be a 'drop-in' but data file formats might need a conversion.
So convert from Minecraft data files to Pumpkin data files. Then it could drop in.
Even if they supply a tool, isn't it drop-in.
Otherwise I'd so no software in existence is really drop-in. Most of them have some update that has to happen.