New answers tagged software
First off, a reminder: The IPTC spec clearly distinguishes "Location Created" (position of the photographer) vs "Location Shown" (main subject seen in image). No camera or GPS unit can set these GPSDestXYZ EXIF tags. They are about a subjective concept. You need software in post-processing to tell what is the "Location Shown". I don't know any software that ...
If you don't mind the rest of your UI being in greyscale, you can switch to an all-greyscale display in OS X without any extra software: System Preferences → Accessibility → Display → "Use grayscale" This will obviously prevent a side-by-side comparison of a greyscale and colour image, but you can at least quickly toggle the display mode. Unfortunately I ...
For JPEG images, you could use IrfanView. It has an option to load images as greyscale. Go to Options menu, then Properties/Settings, then JPEG/PCD/GIF. Then tick the box for Load as grayscale. This setting will be remembered for any images you load. You can use a command line option to specify a different INI file for the settings, so you could set up one ...
All changes in Adobe Lightroom are temporary until you export them. With that in mind I would use Lightroom and select all and convert to grayscale then work through the images as desired. Once done I'd switch them all back out of greyscale with an undo function.
You didn't specify an operating system, but for Linux and friends, geeqie can toggle between normal and grayscale display at a keystroke (Shift+G by default).
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