Can I change the clock on the 6th gen Nano to digital instead of analog?
The 6th gen Nano can now be updated with a software update which introduced the features from the 7th gen, including 16 watch faces, some of which are digital.

Version 1.2 of the iPod nano software brings improved fitness features (which are better able to detect and track walking or running work outs), as well as the new single icon scrolling interface, which replaces the 4×4 icon grid. You can switch back to small icons or turn off home screen items altogether from the Settings menu.
Last but not least, you’ll get access to all the new iPod nano clock face options.