Many users requested this. Simply changing the track configuration from mono to stereo or any multichannel format would be nice indeed 
Still the only chance to get there is to create a new stereo channel and copy the settings. It’s a workaround that can luckily be performed quickly:
https://dl.dropboxusercontent.com/u/38759108/CUBASE%20SUPPORT/turn%20mono%20into%20stereo.wmv
Create a stereo track, pull the events from the mono track down (they stay at their time position when holding Ctrl while dragging/get copied at position when holding Alt + Crtl), in the mixer settings are copied from one channel to another by dragging it there (touched in the labelling area). All settings are identical except the routing which you’ll have to set manually.
Would be much to click the mono/stereo symbol and get a dropdown menu, allowing to change the configuration.