In short, the changes are as follows:
TIGER/WOLF/BEAR -- still required 6 "core" adventure plus 1 "elective" adventure, for a total of 7 Adventures required for Rank. However, many/most of the requirements to complete an individual adventure have been shortened. They did this by adding a sentence at the front of every adventure that reads something like: "Complete Requirements #1 and #2 and three others," or "Complete at least four of the following."
Leaders/Scouts now have more choice, in most instances, of which requirements to complete. They may, of course, complete ALL of the requirements as written. Doing so, however, is optional.
WEBELOS/ARROW OF LIGHT -- Both ranks require completion of the "core" Adventures (5 or Webelos Rank, and 4 for AOL Rank) plus AT LEAST ONE "elective" Adventure. This reduces the previously required number of required "elective Adventures" by one for Webelos Rank and by two for AOL Rank. Thus, total number of Adventures required to be completed for Webelos Rank is now 6 (vs. 7) and for AOL Rank is now 5 (vs. 7).
In addition, many/most of the Webelos/AOL Adventures have reduced requirements WITHIN the individual Adventure by the addition of a sentence like: "Complete Requirements #1 and #2 and three others," or "Complete at least four of the following."
Again, this results in more choice/flexibility for leaders/Scouts.
Additionally, it does not overly penalize a Scout if he can't complete one element of the Adventure for whatever reason. With the modifications, the Scout can (in more instances than previously), still complete sufficient components of the Adventure to earn the rank. This is a good change.
The complete text of the revisions in a comparative "Current vs. Modified" column format is attached for your convenience. THESE CHANGES TAKE IMMEDIATE EFFECT, and should be considered for any Adventures not yet completed for whatever current rank the Cub Scout is working toward (as a practical matter, these changes could also be reflected in any in-progress Adventure, although that is not explicitly stated below). There is NO NEED to purchase new handbooks, simply keep the attached document of modifications.
The modified requirements can be found at this link: http://www.scouting.org/