Luckily it seems MODx generates the URI before the alias, seems strange, but works for me. I guess it makes sense, alias is just 'another/nick' name, where the pagetitle is the real crucial deal.
So '/articles/[[+alias]].html' is useless on two accounts, the second being that the [[*uri]] field does not seem to process other tags (I hope I am completely wrong on this one).
UPDATE site_content SET uri = CONCAT('science-fiction/', uri) WHERE parent = 1000
So this code worked perfectly in my case. I just had to clear the cache, and thing was magic.
Thank you sh0ck23 for the suggestion. I needed it badly.