Player Characters do not have Challenge Rating
As Medix2 mentions in his answer, the Couatl's Change Shape feature is concerned with CR. Player Characters, like those that involve the Variant Human traits, do not have CR, as can be inferred by the description of CR in the Introduction of the Monster Manual (emphasis mine):
A monster’s challenge rating tells you how great a threat the monster is
There are no rules for evaluating a CR for Player Characters. Now, monsters can have racial traits as described in the Dungeon Master's Workshop under NPC Stat Blocks, but Variant Human is not one of the races that can be applied to NPCs using these rules.
While there is no official way to apply feats to monsters and NPCs, you can always apply them by customizing the monsters. When doing this, the CR may change, which you can recalculate using the rules in the Dungeon Master's Workshop for Creating a Monster.
If applying a feat that depends on level, you can use the rules on NPC Stat Blocks for adding races to NPCs for some inspiration. The Dragonborn for example grants the NPC the following trait:
Breath Weapon (use challenge rating instead of level to determine damage)...
This shows one way to determine what would be an appropriate level to use. In your specific case, a Couatl would count as level 4.
It doesn't really matter what level you use to apply the feat. Just choose whatever level would best work for the power increase you want to give to the monster or NPC.