This isn't overly complicated as the enemies, patterns & everything else is the same as a normal mode. The twist is everyone will be randomly switched during the match.
You can be switched to ANY of the characters available to the mode and you can end up with doubles, as many times during this we had doubles, sometimes to our advantage like two Torbs and others not so much, like two Widows.
Your ultimate meter carries over during any switches, so if you're a character with less than ideal ultimate or the situation doesn't call for it, you can save it for the next swap.
The swap happens wherever you happen to be on the map, so be prepared to be pretty mobile during this, as an example I started as Soldier 76 & was covering the top left stairs when I switched to Zen, which isn't an ideal spot for him all alone.
I think the swaps are set to a timer, though I can't say for sure. I'm going off memory here, but I believe my order went --> Soldier, Zen, Baptiste, Torb, Soldier, Baptiste, Brigitte, Soldier & Tracer.
As long as you have a pretty good grasp of all the characters (which really after 4 years, most probably do) and a team who knows the mode, it shouldn't be a problem. The only trouble could be really bad swap luck, like ending up with 3 healers during the final boss wave.