I would want to be able to hit a few buttons to change which mic is in "lead mode". To that end, I would try setting up a submix or subgroup that any channel can be assigned to. On that sub, I'd either set it up as the "lead" sub, meaning it would add a bit of gain and some brighter EQ, or I'd set it up as a background sub with things a bit more muted.
If it's a "lead" sub then I'd add the current lead channel to that group while still leaving it in the overall vocal group or mains group, so it would goose the lead signal a bit. The group assign buttons would now be simple on/off switches for which mic is in lead mode.
Or, if it's a background group, then all the mics go to that group only, and the lead mic is also assigned to go straight to the mains. You change leads by turning on/off the send to mains. I kinda like the second option best since it will still be nice to have a fader for the whole group and you'll be adding back in a less-processed version of the lead as opposed to applying more processing to the lead.
In general, channels come forward when they have more level, panned center, brighter eq (more highs), longer reverb pre-delay (yes, that's longer as in farther from the imaginary back wall) and/or less reverb mix. Channels go farther back with the opposite on all of those.