That's not an answer.
you cannot do that with InDesign.
At least not without creating new graphics.
I'd use a batch process with PhotoShop to convert from CMYK to Grayscale.
And update the links in the InDesign document after the conversion.
If you have trouble doing batch processes with PhotoShop ask in the PhotoShop forum:
Are you sure we cannot do that with InDesign? We already did. Thanks for your good advice and excellent answer
you could do it, but I would not recommend it.
Export a placed image to a grayscale JPEG using InDesign.
Yes, that's possible.
But it would not be the same image data compared to the original. Never. One obvious example would be a rotated image or an image that is stretched. InDesign would create a new pixel structure the best it can. But it would not be the original.
And e.g. clipping paths would not be maintained. There are other problems with JPEG quality as well.
How about a TIFF or a PSD ?
InDesign would never be able to export to TIFF or to PSD.
So the best thing is doing this with PhotoShop.
And PhotoShop is really good with batch processing.