One of the best feature of the APS format is to be able to change film in mid-roll, on the fly. The APS camera automatically rewinds the film into the cartridge and when you re-insert it, it automatically spool it to the right frame.
Unfortunately, this feature is generally unimplemented in low-end point-n-shoot cameras. The Canon Elph Jr. is a perfect example. The Elph Jr. is an excellent camera with its fix-focus 26mm f/2.8 lens. It is small enough that many photographers, including myself, carry it everywhere. It supports the mid-roll rewind, yet, it lacks the mid-roll change feature.
When you press the mid-roll rewind button, the film is re-wound into the film cartridge and the visual expose indicator is set to the exposed setting.
Trying to load an exposed film cartridge back in the Elph Jr., or other APS camera, will fail. However, by turning the film cartridge visual indicator to unexposed, I was able to re-load the partially exposed roll of film in my Canon Elph Jr. camera. (Remember to turn the film cartridge clock-wise, following the arrow.) The Elph Jr. thinks the film cartridge is unexposed and wounded the film to the first frame. I simply turned off the flash and kept pressing the shutter release, in the dark, to manually wind it to the frame where I left off. You can do this in a window-less bathroom at night, or get a film loading bag. Just remember to turn off the built-in flash!
Note that the above procedure is not recommended by the camera manufacturer. Doing so may void your warranty and damage your camera equipment. I can not be held responsible for your actions.
Now I am able to load different film speed in my Elph Jr. without worrying about losing unexposed frames.