Basically we could "easily" add a paint brush which replaces block textures. The necessary game mechanics are already there (accessible in the creative mode F8 tools)
I'm only struggling with finding a reasonable way to change the material in survival mode... it feels a bit weird if a wooden block could be turned into a metal block just by using a paint brush, for example
I think we need another way or another tool for that... maybe a hammer & chisel? It could be a dedicated tool then to replace existing blocks in the world with another block (you just need the new block in your inventory). What do you think about that?
Why not just restrict it to only being able to change material for same material, that way it prevents people from exploiting it. Depending on how you have things set up you might have to create a couple of tools for survival like "wood chisel", "Stone Chisel", etc. so you can restrict the materials, but hey you don't use wood chisels to carve stone in real life so no big having to have a couple of them in game for the right job.