There's been some confusion on how exactly the LFR Year 3 adventure tier/level group system works, so here's the official word from the boilersplates with boldvision™ for what I think is important to point out:
Heroic & Paragon Tier LFR Adventures
This adventure is written to support the entire Heroic tier of
play (levels 1-10) or the entire Paragon tier of play (levels 11-20).
Any character of the appropriate tier may play the
adventure, but the highest-level character in the party
must be within three (3) levels of the lowest-level
character in the party.
Living Forgotten Realms defines five adventure levels
within each tier (the even numbers). The choice of
adventure level affects the difficulty of most obstacles
(particularly combat encounters and skill challenges) the
PCs will face during the adventure, and also determines
the rewards available to the PCs for successfully overcoming
those obstacles. The players must decide as a group
which adventure level they want to play, and inform you
of their decision before beginning the adventure.
We recommend that you calculate the average character
level and use that as a starting point, but a group of
experienced players might choose to "play up" for a
greater challenge, while a group of less-experienced
players might choose to "play down" while they're
learning the ropes.
The group may not choose an adventure level that is
more than three levels above the lowest-level character
in the party. For example, a group with a level 2, level 3,
and four level 5 characters may not choose to play the
level 6 version of an adventure because level 6 is more
than three levels above the 2nd-level character. This
group could choose to face the adventure at either level
2 or level 4, but they will probably choose level 4
because that's the best fit for the group (the average
character level is 25/6 = approximately 4).
If (and only if) the group can't agree on an
adventure level, the DM may cast a tiebreaking vote.
Epic Tier LFR Adventures
Each adventure is run at a specific level. For the purposes of the rules below, a level 21 Epic adventure is used as an example.
This Epic Campaign adventure is designed for level 21
player characters (PCs). Players may only play this
adventure with characters that have reached 21st level
through play in the Living Forgotten Realms campaign. To
be clear, it is okay to use a character that was started
above 1st level following the rules in the LFR Campaign
Guide and subsequently reached 21st level through the
play of LFR adventures. However, the campaign rules
limit new characters to a starting level of 11. It is not
legal to create a brand-new 21st-level character
specifically to participate in the Epic Campaign.
If a character has reached 21st level but is not the
correct level for this adventure, which can only happen
by playing the Epic Campaign adventures out of order,
that player may adjust the level of his or her character
accordingly for the duration of this adventure.
Adjusting Level Up: If a character is below the
designated level for this adventure (but has reached at
least 21st level), increase the character’s level to the
correct level for this adventure, following the core rules
(so the character gains powers, feats, ability score
increases, and so forth as normal). The player may not
change any of the character’s existing magic items, and
the character gains no new magic items.
Adjusting Level Down: If a character is above the
designated level for this adventure, reduce the
character’s level by removing all benefits gained from
higher levels. The character may not use any of the
magic items or Story Awards gained from Epic
Campaign adventures designated for levels higher than
this adventure. For example, if a player had to skip the
level 21 adventure but has already played the level 22
and level 23 adventures, that character may not use any
of the magic items gained from those adventures during
the lower-level adventure. Anything that happened in
the higher-level adventures is temporarily “unwound,”
as if the character had played the adventures in the