I've always been pretty lenient regarding players changing their characters as a result of new books/erratta - whenever something comes out that changes their character (martial character gets new options from MP2, erratta nerfs one of cleric's powers) I've given a free retrain. Then again, I know with my group that they won't abuse it and have so far only made changes that fit their character concepts.
In the campaign I play in (same players, just a swap of DMs), the DM let me change from Warlock to Sorceror when PH2 came out - the character had been a 3.5 Sorceress so the swap actually took her closer to her character concept (plus, I was getting more an more annoyed with the Warlock as a class). In the same campaign, the rogue may be changing to a monk (rogue doesn't play like he wanted it to) and the str cleric may change to rune priest (cleric of creation AND destruction - rune priest mechanic really catches that). I see allowind similar swaps in my own campaign if the essential classes really catch what the players wanted... though I doubt it will happen.
I also intend to allow races that gain an optional stat boost (so, if Dwarves go from Wis & Con to Wis & Con/Str) to take the new option.
Going into the campaign, we agreed to use full erratta and avoid house rules(we do have a few, like free expertise feets and 1/tier NAD boosts), so any general rule changes from essentials will be incorporated.