that's odd, I checked on jisho and indeed
河 is under "Other forms" for
川, while the discarded mahjong tile should be ホー (though now that I notice, that has かわ as well under other forms)

So that's seems something that can be adjusted, since is working correctly on jisho.
Regarding the other terms changing,
@htatsuha I've searched for
妹さん on goo and kotobank and I found no entry for it in there as well...what is missing exactly?
Also I think you are ignoring all the terms that got improved,
凍結 to randomly mention one.
Old
凍結 definition was :freeze (ex. program,food, etc...)
New is: freezing (e.g. water); freezing (prices, wages, assets, etc.), moratorium, suspension (e.g. investment)
The new one seems better to me.
Anyway I tought several people could end up annoyed with the changes in "keyword" confusing them during the quizzes, but I think overall is a win because of the majority of updated definitions.
If reverting back to the old dictionary would be the choosen route in this case, would be possible for renshuu to "host" several dictionary versions and let the individual user choose which one to use?