I may have missed it but I know of no quick way to convert tracks/channels, including instruments and groups, between mono and stereo (etc). It seems such a basic requirement but the only way I know how at moment is to create a new channel from scratch and copy the settings over.
Edit: it’s even more of a pain if there are automation tracks to be copied over, as I have just been reminded!
Turning the current mono/stereo symbol into a simple toggle would be a more elegant solution.
Please vote if you agree. Cheers, C