When I'm in riichi and draw a red 5 from the wall, am I allowed to exchange it with the non-red 5 in my hand?
I would argue it might be allowed, because it does not change the yaku, wait or hand structure in any way.
In case the above is correct, what happens in case there is an (isolated)
678 in my hand? Would I be allowed to discard the 8? Given the above rationale, the answer should be yes. However, this could be very complex in practice if the
678 were not isolated, or if there are were similar sequences in other colors, and I doubt this would be acceptable for the players to keep exact track of.
In comparison: I've learned earlier that a kan is allowed, when there are 3 equal tiles already present.