I recently ordered some spare E3D 0.4 mm nozzles. However, there was a mix up at the factory and they delivered 0.2 mm nozzles instead.
I usually print at 0.3 mm using a 0.4 mm nozzle. Can I still use the 0.2 mm nozzles to print at a resolution of 0.3 mm?
To get the best results, the plastic coming out of the nozzle needs to be squished/ironed down by the nozzle. If you are using a higher layer height than nozzle size, this does not happen. Instead, you are taking a 0.2mm diameter string of plastic and folding it back onto itself to create a thicker 0.3mm bead.
Triffid_Hunter's calibration guide recommends using a layer height that is not higher than 80% of your nozzle size, so with a 0.4mm nozzle you should not print layers thicker than 0.32mm, and with a 0.2mm nozzle you should not print layers thicker than 0.16mm.
While it is not impossible to print thicker layers with a smaller nozzle, the results won't be as good. Moreover, if you are used to working with a 0.4mm nozzle then swapping to a 0.2mm nozzle may take some getting used to, as printing with smaller nozzles is more difficult (for example, the smaller the nozzle, the higher the likelihood of clogs).