One doesn't just add "allow_inheritance=True"

mongoengine_allow_inheritance_be_careful.py

from mongoengine import Document, StringField, ListField, EmbeddedDocument, EmbeddedDocumentField


# ******** before remodel: **********

class FailedChargeAttempt_Old(EmbeddedDocument):
    id = StringField()
    failed_reason = StringField()


class ChargeModel_Old(Document):
    attempts = ListField(EmbeddedDocumentField(FailedChargeAttempt_Old))


charge_old = ChargeModel_Old()
charge_old.attempts.append(FailedChargeAttempt_Old(id='blah', failed_reason='extended data'))


# ******** after remodel: **********

class BaseChargeAttempt(EmbeddedDocument):
    meta = {
        'allow_inheritance': True
    }

    id = StringField()


class FailedChargeAttempt(BaseChargeAttempt):
    failed_reason = StringField()


class ChargeModel(Document):
    attempts = ListField(EmbeddedDocumentField(BaseChargeAttempt))


# ******** let's test *************

old_son = charge_old.to_mongo()
charge = ChargeModel._from_son(old_son)
print(charge.attempts[0])
print(charge.attempts[0].to_mongo()['_cls'])
print(charge.attempts[0].failed_reason)