Best way to individually manipulate a scenes mesh instantiated by GridMap

Godot Version

4.3

Question

I started a week ago building my proof of concept for a 3D game. Now the core mechanics work and i am starting to build environments with GridMaps and have built a mesh library that has all my objects as a scene in it and is working as intended.

One of my objects is a sign with a placeholder texture.

What would be the efficient way of targeting specific instances of this sign scene on the grid and replace only the one texture. The area i want to replace is already its own material and basically only a matter of referencing another texture file.

Currently i cant wrap my head around on how to reuse the sign scene as much as possible, so in theory i can alter the signs structure itself in the future.

Any hints in the right direction are appreciated.