Close to three years ago I wrote about using Oddly Specific Objects' alternate "motherboard" to modify a classic Casio F-91W watch: Pimping my Casio with Oddly Specific Objects' alternate motherboard and firmware. That blog post goes into the detail of swapping out the guts of the Casio and building and uploading the firmware. 

Happily, Oddly Specific Objects is back with a "Pro" version of their alternative Casio internals which now features an accelerometer and an alternative LCD. The original Sensor Watch used the existing Casio LCD display but the custom LCD allows for more complicated text to be displayed on screen. It's not a dot-matrix display so there are lots of limitations but it's still a fun upgrade.

Also, the Sensor Watch Pro, as it's called, requires no soldering (unlike the original version). Here's one I prepared earlier (that is a Casio F-91W with new internals):

You can see the new display in action there showing the day as FRI. As with the original Sensor Watch there's a browser-based emulator that uses emscripten to get the watch running on your computer. This is pretty important because flashing new firmware to the watch requires dismantling it.

Since there's no soldering the process of upgrading your Casio is simply take it apart and put back in the parts that came in the kit (along with a small piece of metal that's transferred from the original Casio watch to make the battery connection):

Since I bought the optional accelerometer add on it's necessary to put in place a little piece of caption tape (you can see if covering the five pads on the image above) and the insert the accelerometer board itself:

The five vertical traces at the top are the micro USB interface used for flashing the firmware.