As far as River is concerned - They still spend a year in B in which they'd most likely get a promotion anyway, no?
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.
somebody correct me if I am wrong on the above.