mongoengine: change document subclass

Question

I want to migrate an old mongoengine schema to a new one.

Currently, the schema looks like this:

class Main(Document):
  field_m = StringField()

class MainSub(Main):
  field_ms = StringField()

class A(MainSub):
  field_a = StringField()

class B(MainSub):
  field_b = StringField()

class C(MainSub):
  field_c = StringField()

The target schema is:

class Main(Document):
  field_m = StringField()

class MainSub(Main):
  field_ms = StringField()

class A(MainSub):
  field_a = StringField()

class SpecialSub(MainSub):
  field_ss = StringField()

class X(SpecialSub):
  field_b = StringField()

class Y(SpecialSub):
  field_c = StringField()

where all instances of B and C are to become instances of X and Y, respectively.

There are multiple references from other subclasses of Main or Document to the A, B, and C classes (which will be references to A, X, and Y, repsectively).

For my project, I have a migration module which runs independent from the rest of the code, and should not import the actual models itself. I do not have much knowledge of MongoDB myself. I see three possible solutions:

A) write a schema which mixes both old and new for the migration, then copy each object, update all references, and then remove the old objects (very complicated due to references)

B) change the subclasses using MongoEngine. How can I do this?

C) change the subclass by directly using MongoDB. How can I access MongoDB directly via mongoengine, and what would the respective MongoDB query look like?