One can already place machines above conveyor belts, replacing them in the process. Here are some other things that could do something similar:

• splitters over other splitters (copy orientation, just swap out splitter type)
• machines over machines of the same type (just copies the config as a way to implement https://modulus-feedback-and-suggestions.featureupvote.com/suggestions/657117/copy-building-config)
• advanced machines over their basic counterparts, if there is space (preserving the configuration)
• basic machines over advanced machines, (adding conveyor belts, downgrading the configuration if possible)
• maybe also something with cosmetics

I am unsure if it is better for this replacement to copy the orientation of the old machine or if it should instead use the orientation of the "held" hologram of a machine. Perhaps one behavior by default and the other by holding shift?