iirc, not necessarily - demotion and promotion in argentina is done based on a team's average performance over 3 seasons; a newly promoted or demoted team has only their last season - since promoted/demoted - counted in that.
the first 2 teams in B are promoted straight away, the 3rd, 4th play play-offs
all in all, iirc, River has to do really, really well in order to get immediately promoted after only 1 year in B.
interesting ... I saw this in Clarin but wasn't sure - bad translation - if it was just some proposal or everything was done and dusted.
Either way, they still get to spend at least a season in B, unlike those lucky bastards from Inter back in the days.