This is something of a technical idiom. If you have a system of parts (e.g. a home computer), and you can pick up one module (e.g. a RAM stick) and "drop in" a new one in its place without fiddling about (because the connections are the same and the two sticks are the same size), then the new RAM stick is a "drop-in replacement" for the old one.
The phrase has since been transplanted from the physical realm into the virtual one, where a piece of software or a piece of code or really anything modular can be "dropped in" to where you took out the original thing, without significant alteration or reconfiguration.
So, it expresses a one-shot, effortless replacement of a part.
In this case, the syntax of the two LaTeXes is similar enough that you can switch over without having to update your code.