Function convert_to 

pub(crate) fn convert_to(
    from: &dyn Mob,
    entity_type: EntityTypeRef,
    params: ConversionParams,
    after_conversion: Option<&AfterConversion<'_>>,
) -> Option<SharedEntity>

Unlike vanilla, inserts the new mob before transferring state so rejected insertions leave the source, passengers, and leash untouched.