@pilotfred
If you only want to do this temporarily, you could simply set pans at centre and patch only the left (or right) of the mix to an output socket.
Where this wouldn’t work is with stereo input channels, the workaround being to use the stereo width control to mono them.
BUT – No you shouldn’t have any issue altering the mix config before loading up scenes. Channels will be sending at the same level and the all mix processing will remain the same.
Just be aware that once you store scenes again, you will lose any panning data from before. You will probably also want to go over your output patching as if you patch sequentially, you’ll have gaps from all the Right channels that don’t exist any more (the left channels will become the mono output).
Thanks,
Keith.