The level range of the adventure was determined based on what level the 4E versions of the monsters in the original adventure are. This was done with an eye toward using as many of the same monsters as were used in the original as possible.
Obviously, though, some substitutions came into play. Also, since 4E encounter design is quite different from early edition encounter design, some encounters were changed significantly, to fall more in line with 4E design precepts.
But, in general, it's a pretty faithful adaptation, often in actual specifics, and when that's not possible, at least in spirit.
Shiv