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Something else I haven't seen mentioned yet: many upright pianos do not have a proper 'una corda' pedal.
Typical pianos have between 2 and 3 strings for each note, all tuned to the same pitch, so the sound is full and more easily heard. The hammer typically hits all these strings with each key press.
On a grand piano, the leftmost foot pedal moves the actual keyboard and hammer assembly slightly to one side so that only a single string for each note is struck (una corda = single string). This results in a sound more like that of a pianoforte which is almost harp-like in sound.
On many upright pianos I have seen, the left pedal simply lowers a felt strip in between the hammers and strings making the sound quieter. While this is similar, it may not be what the composer had in mind for the particular piece.