Levelers
The Rise of the Eldrazi FAQ is up, so we can now see the exact wording of the leveler rules. I'll quote the relevant portion for us coders:
The question of whether or not leveling up removes previous abilities is answered, which makes it possible to actually implement these, now. However, nothing says it's not possible to put more than one "Level N+" box on a single card, which I think we can expect in the future (maybe a few years down the road) as an extension of the leveler mechanic.
(The rules for the "level up" keyword itself not included because we all know what it does... stick level counters on it.)710. Leveler Cards
710.1. Each leveler card has a striated text box and three power/toughness boxes. The text box of a leveler card contains two level symbols.
710.2. A level symbol is a keyword ability that represents a static ability. The level symbol includes either a range of numbers, indicated here as "N1-N2," or a single number followed by a plus sign, indicated here as "N3+." Any abilities printed within the same text box striation as a level symbol are part of its static ability. The same is true of the power/toughness box printed within that striation, indicated here as "[P/T]."
710.2a "{LEVEL N1-N2} [Abilities] [P/T]" means "As long as this creature has at least N1 level counters on it, but no more than N2 level counters on it, it's [P/T] and has [abilities]."
710.2b "{LEVEL N3+} [Abilities] [P/T]" means "As long as this creature has N3 or more level counters on it, it's [P/T] and has [abilities]."
710.3. The text box striations have no game significance other than clearly demarcating which abilities and which power/toughness box are associated with which level symbol. Leveler cards each contain only one text box.
710.4. Any ability a leveler card has that isn’t preceded by a level symbol is treated normally. In particular, each leveler permanent has its level up ability (see rule 702.84) at all times; it may be activated regardless of how many level counters are on that permanent.
710.5. If the number of level counters on a leveler creature is less than N1 (the first number printed in its {LEVEL N1-N2} symbol), it has the power and toughness denoted by its uppermost power/toughness box.
710.6. In every zone other than the battlefield, a leveler card has the power and toughness denoted by its uppermost power/toughness box.
In other words, you could interpret Coralhelm Commander's text box as though it said the following:
2/2
Level up({1}: Put a level counter on this. Level up only as a sorcery.)
As long as Coralhelm Commander has at least two level counters on it, but no more than three level counters on it, it's 3/3 and has flying.
As long as Coralhelm Commander has four or more level counters on it, it's 4/4 and has flying and "Other Merfolk creatures you control get +1/+1."
The question of whether or not leveling up removes previous abilities is answered, which makes it possible to actually implement these, now. However, nothing says it's not possible to put more than one "Level N+" box on a single card, which I think we can expect in the future (maybe a few years down the road) as an extension of the leveler mechanic.
({1}: Put a level counter on this. Level up only as a sorcery.)