I've had mixed experiences reading gender-bender manga.

The better ones showed that gender is a social construct - that gender plays no role in determining who a person is or what they are capable of, and that preferences, hobbies, behavioral patterns... none of these are influenced by a person's gender, but rather by society's perception of gender and its need to assert gender roles.

The worse ones were quick to impose gender roles, swiftly establishing the Do's and Don'ts of being male or female, mercilessly confining their characters to a strict set of behaviors 'appropriate' to their respective genders.

Kanojo ni Naru Hi belongs to the latter.