I'm not sure why it was changed. Most likely an aesthetic decision. Ever get stuck behind a pre-LCi and puzzle at how "sucked in" the rear tires look? This is worst on non-sport pack cars due to the small tires and elevated ride height. It looks very "economy car" to me and I'm sure BMW stylists thought it was really ruining the look of their car.
As far as I know the difference is in the hubs but I have never owned a pre-LCi so I really have no point of reference. Don't know if these are interchangeable or not.
As to running 19 x 10, you should be able to as long as the offset is where you need it. I think Avantgarde sells M359 style wheels that are 19 x 10 et 38 that work on LCi cars. I could be off on those numbers because I am just quoting from memory but look into it. You should not need to change hubs to run 10" wheels.