Multiple serializers for a single model

I have a User model that hasMany Reminders.

When a User is first created, I want the reminders to be embedded. When it is updated, I don't want its reminders to be embedded. How can I do this?

My strategy was to create a custom new-user-serializer that had DS.EmbeddedRecordsMixin and

reminders: { embedded: 'always' }

and use that in my UserAdapters createRecord method, but I couldn't get it working.