If you need to change the sound to the harpsichord as quickly as drawing a stop then you need an organ which includes the harpsichord samples. (Loading any sample set to replace the current instrument is going to take longer that a couple of mouse clicks.) You could create one if you used the custom organ facility with HW and had a suitable set of samples. You might consider an instrument which includes a harpsichord such as https://www.sonusparadisi.cz/en/organs/harpsichords-etc/claviorganum-sonus-paradisi.html.
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