Change inchikey directly
According to the InCHI FAQ, the 2nd block encodes "encodes stereochemistry and isotopic substitution information". So if you want to generate the corresponding InChIKey for the case where there is no stereo present nor isotopes specified, then it looks like this is possible. If you also want to force the neutral form, you could try setting the final character to 'N' (it may instead be P or O).

Your mileage may vary. To be honest, I wouldn't do this if I were you. I would generate a SMILES without isotopic or stereo information (or remove this programmatically), and then get the InChIKey of that. This would avoid any nasty surprises.