(There might be a technical reason that makes this difficult to support that I don't understand, like a race condition or state integrity problem, in which case, fair enough, but otherwise, I think from a design perspective this should be allowed.)
Currently, once a tour starts, any custom rule setting become locked in and are no longer possible for drivers to change. Because of this, if a problem with the tour, such as outdated bans or some mistake in rule interactions (e.g. -Rayquaza-Mega versus Mega Rayquaza Clause), is discovered after this point, there is no option but to either destroy and remake the tour from scratch, or continue with it in a wrong state. I have seen situations like this occur many times, and it is always awkward to deal with, even if no battles have started yet, since with a restart all users need to rejoin, the bracket state is regenerated, etc. It would be better to instead allow drivers the opportunity to fix mistakes identified during such tours by being able to use /tour rules even after the tour has started.
Supposing that this is currently disallowed by design, I tried to think of reasons why that might be the case, but I couldn't think of a compelling one. The first reason I considered was that allowing this could enable accidental changes to be made to the tour rules after starting. This is true, but a. it also would be possible to revert them again afterwards and b. this can already occur immediately before the tour begins with no such way to mitigate the damage. Another objection might be that it could be seen by users as unfair for rules to change during a tour. This also may be true in some cases, but drivers already are entrusted with other tools that could be used in an 'unfair' way to a much greater extent, such DCing users from the tour entirely, because those tools also have legitimate uses. I think drivers should be trusted to use this functionality properly, and explain to users why they have changed rules (e.g. overlooked bans, etc) rather than being hard locked out of it. If these objections or anything similar have been reasons to restrict this functionality by design, it seems like having a "lock mode" on the tour would be a better way to address them rather than completely removing the custom rule function after a tour starts.
Currently, once a tour starts, any custom rule setting become locked in and are no longer possible for drivers to change. Because of this, if a problem with the tour, such as outdated bans or some mistake in rule interactions (e.g. -Rayquaza-Mega versus Mega Rayquaza Clause), is discovered after this point, there is no option but to either destroy and remake the tour from scratch, or continue with it in a wrong state. I have seen situations like this occur many times, and it is always awkward to deal with, even if no battles have started yet, since with a restart all users need to rejoin, the bracket state is regenerated, etc. It would be better to instead allow drivers the opportunity to fix mistakes identified during such tours by being able to use /tour rules even after the tour has started.
Supposing that this is currently disallowed by design, I tried to think of reasons why that might be the case, but I couldn't think of a compelling one. The first reason I considered was that allowing this could enable accidental changes to be made to the tour rules after starting. This is true, but a. it also would be possible to revert them again afterwards and b. this can already occur immediately before the tour begins with no such way to mitigate the damage. Another objection might be that it could be seen by users as unfair for rules to change during a tour. This also may be true in some cases, but drivers already are entrusted with other tools that could be used in an 'unfair' way to a much greater extent, such DCing users from the tour entirely, because those tools also have legitimate uses. I think drivers should be trusted to use this functionality properly, and explain to users why they have changed rules (e.g. overlooked bans, etc) rather than being hard locked out of it. If these objections or anything similar have been reasons to restrict this functionality by design, it seems like having a "lock mode" on the tour would be a better way to address them rather than completely removing the custom rule function after a tour starts.