Am I allowed to take a red 5 after reaching?

Question

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.

Answer 1

I think most "official" rules will not allow you to do this. (For reference, I looked up the Japanese Mahjong rules from the European Mahjong association here.)

In principle, I think swapping tiles that do not change the wait structure would be allowed, but it is impossible to verify. In your example of swapping the 5 for the 8, all that is visible is the discarded 8. From an outsider perspective, it is possible that you completed a different meld and simply discarded an 8 from 5-6-7-8 in hand.

It also appears that the hidden kan/kong is allowed, but only when the pung is isolated and cannot be interpreted as a different set of melds (e.g., you cannot draw and complete a kong if you have 1-1-1, 2-2-2, 3-3-3, because those tiles could be interpreted as 1-2-3, 1-2-3, 1-2-3).

Answer 2

According to the Japanese Wikipedia article on riichi:

打牌の選択（手牌の入れ替え）ができない。つまり、和了する場合と後述の暗槓する場合を除いて、自摸した牌をそのまま捨てなければならない。

Translating, this means "You are no longer allowed to select your discards (switching the drawn tile with a tile in your hand). In other words, save for drawing your winning tile or calling a concealed kan as described below, you must discard each tile you draw."

That means if you draw a red five after calling riichi, it's either your winning tile and you're calling tsumo, you have three other fives of the same type and you're calling a concealed kan, or you're discarding it. You cannot simply switch a red five with a regular five, even if it wouldn't change your waits.

Answer 3

Japanese mahjong does not allow swapping tiles into the hand after riichi, as noted by so many.

Apocryphally, the European Mahjong Association did allow this between 2008 and 2012 (changed that year) because their community's comprehension of the rules as played elsewhere was incomplete and lacking, so they made stuff up to bridge the gaps. Now, EMA rules from 2016 onward are functionally the same to what people would consider to be modern Japanese mahjong, with variance only related to protocol and game etiquette, with a natural focus on a competitive style.

As for using Japanese wikipedia or a pro organization or a jansou chain as a source: it depends who, for what, and under what optics. Using a single source might not always work, but if they all conform to one or two clear options, then it does make sense. There are people who have their done their research on this and other topics already. Some things do have variance: this isn't one of them. House rules govern agreements between 4 players that have chosen to abide by them, but this does not make them normalized or "an equivalent option" because they simply are not common or relevant enough to make that distinction.