<p>As an extension from <a href="https://3dprinting.stackexchange.com/questions/264/when-to-use-1-75mm-vs-3mm-filament">this</a> question, is there any reason that you would not be able to use 1.75&nbsp;mm filament in a printer that takes 3mm filament?  I know you would have to change the filament size in the slicing of prints but would there be any other problems?</p>

<p>Also, would using 1.75&nbsp;mm filament be possible if the nozzle diameter was greater than 1.75&nbsp;mm but less than 3&nbsp;mm?</p>

It may work for a short time but you're going to fill the melt chamber quickly and possibly overflow to a point where the filament isn't constrained causing a messy jam.  All the molten plastic will likely flow backwards to a point where the diameter isn't 3mm any longer (probably next to your drive gear) but depending on the length it may just flow up, cool down and jam the extruder.
No, the filament would just pass through unheated and not do anything.