Hi, welcome,

In MJPEG mode you can modify the resolution of the video the camera captures and push it over 1920x1080 and also manipulate frame rates, but the general opinion is that AVCHD produces better quality than MJPEG mode, sharper footage with less noise. A quick search on this site will show you many tests that people did so you can see for yourself whether it works for you or not.

Now if you want to upscale your video in postproduction that is a different ball game. Some dedicated software for that exists out there and you should search for best practices for upscaling from HD to 2K. If you have a sharp lens, good 1080p24 settings and some free time you can push those few pixels up if you absolutely need 2K.