-The MM1 Hydra won't be errated. Instead things like the new version of the Hydra is printed in the MM2 and it is assumed that if you want a Hydra fixed, you'll just use the one in the new book
That's a bit harsh. It's not that the MM 2 hydra replicates the MM 1 hydra except for fixing its hp and damage output when bloodied. There's plenty of other differences, both mechanical and flavour-wise, between the two.
I've recently submitted a lengthy review of MM 2 (it's in German but people will be able to read it on dnd-gate.de in a week or two) and I went over both books very carefully. While there are a lot of MM 1 monsters in MM 2 (angels[FONT="], predatory vine, beholder, troll, cyclops, hydra, eladrin, gnolls, gnomes, goblins, hobgoblins, humans, oni, shadar-kai, troglodytes, warforged) [/FONT]no single entry reads like an errata. As with the hydra, the MM 2 entries show a lot of mechanical innovation (well beyond fixation) and variation in flavour.
What's more, the elegance and simplicity of 4E vis-a-vis 3.5 means that you can correct MM 1 monsters "on the fly" if you wish, the official recipe for solos "the MM 2 way" being:
[FONT="]20% fewer hit points, –2 to defenses, increase damage output by 50% when bloodied.[/FONT] (Fans produced more accurate versions than this, but this one has the benefit that you can work it out in 30 seconds.) Using that recipe on the MM 1 hydra will feel different to anything that MM 2 offers, so no, MM 2 has far from rendered MM 1 obsolete.
I whole-heartedly recommend people to get the MM 2, but precisely because it
complements the MM 1 so well, not because it supplants it.