Can you print at 0.3 mm with a 0.2 mm nozzle?
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).
And the print time will be much longer.... You only get a quarter the plastic at half the diameter, and you have to make many more passes. Your feeder may complain/skip/chew when trying to feed at 0.3 rates into a 0.2 nozzle, as well. That's 2.25 times as much plastic as the nozzle would be expected to feed.