1. Review help page and
2. documentation links: DONE
3. Write new tests for Version > Drupal8.5 advance test cases.

2. Write documentation for complex upe Like :
Link : 
drupal.org/docs/8/modules/entity-update/advanced-use-for-complex-updates

------------------- Advanced use --------------------
Update entity structure update or multiple operations
-----Developers Only - Not for Production sites.-----
-----------------------------------------------------
If you want to change the structure of an entity type (Example : make non
translatable entity to a translatable) and your entity has data, you can try
the following steps.
CAUTION :
- You must structure the entity before starting the developments.
- This type of operations must not produce on production sites.

1. Cleanup the backup data table.
drush upe --clean
2. Backup data of your entity.
drush upe ENTITY_TYPE_ID --bkpdel
3. Update the code (Entity type definitions for example).
UPDATE YOUR ENTITY TYPE STEP BY STEP (See the doc).
4. Update the entity type (No need to backup full database again).
drush upe ENTITY_TYPE_ID --nobackup
-> Note : You can use 'gunzip < backup_XXX.sql.gz | drush sqlc' if necessary.
5. Create entities from entity backup database once every things are success.
drush upe --rescue
6. Cleanup the backup data table once every things are success.
drush upe --clean

User case 1. make translatable entity to a non translatable
1. Remove 'translatable = TRUE,'
2. update entity
3. Remove language key Ex : '"langcode" = "langcode",'
4. Update entity

User case 2. make non translatable entity to a translatable
1. Add 'translatable = TRUE,'
2. Add language key Ex : '"langcode" = "langcode",'
3. Update entity