Basic D&D levels might not correspond to 5e levels exactly. The conversion guide linked in post #2 says that older edition character levels (except 4e) are equivalent to 5e levels. I can believe that for 3.5e, but I would have guessed that a Basic D&D module for level 1-3 characters would translate to level 1-5 for 5e. I would probably allow the group to level quickly for the first couple levels, which might result in later encounters converting a little closer to the original versions. I never played B2, so I don't know if PCs were expected to level along the way.