Even if they changed voice casting for pre-existing characters, it wouldn't be that difficult for new voice actors to try and match their predecessors in voices somewhat and let the sound techs 'sort it out' - they certainly have had the tech to do it for some time now.
Having said that... Bioware has a tendency to pay a lot of attention to details when it comes to Mass Effect, so this would be a relatively minor 'problem' to overcome - but realistically, I don't think there will be many changes that would impact the game in a negative capacity (least of all one that would be noticeable unless you are really looking to spot the changes in question).
Anyway... I'm replaying Mass Effect 2 myself right now with all the DLC content and having a blast.
I thought about simply importing my save from previous ME2 game, but it was corrupted (unlike my ME1 save) so I decided to replay until the third game comes.
I gotta say I love the ME series... and (finally) there should be same-sex relationship options in this third release.
Heh... either way, will be interesting to see how the story comes to a wrap and what might happen after ME3 - if they perhaps continue it in some shape/form.
I would love it if the ME movie for example would be done 'properly'.
I find a lack of genuine SF and technical aspects that could easily be realistic in real life dismissed as 'techno babble' that is usually replaced with hardcore action and explosions.
I see no reason not to have all those aspects combined.