So, they've stated that they'll use a simple formula to figure out the leveling squish. Essentially, if a level 120 is going to squish to a level 50, that's dividing by 2.4. I'm not sure if it will be that simple, but let's assume it will be for the purpose of this post.
Currently, I'm leveling up 8 alts. They're all in the mid to high 40s. I've been taking it slowly with them during my down time from playing my main. Anyway, that means I'm about 75% of the way through Eastern Kingdoms or Kalimdor. After the squish, my alts should move down to about level 20. There's not going to be enough content in the revamped WoW zones to finish leveling up to 50. I guess I can't propose that they leave 1-50 levels the same without squishing, because that would allow people to level up to 50 now and basically get the equivalent of a boost.
In other words, characters who are in the middle of leveling now are going to be left a little bit on their own so that they'll likely have to do a partial expansion to get to 50. This would be the same in any of the expansions. There's no way I see around it, and I don't see it as a true problem. However, I figured I'd point that out to people.