Vtiger 5.x based entity module that followed Module Manager compatibility (vtlib API) would
continue to work on Vtiger 6 without changes.
You will need to review the custom code and refactor them to get it work on the newer
platform.
Find more details at - [Package Structure](https://code.vtiger.com/vtiger/vtigercrm-manual/wikis/Developer-Docs/Develop-Extensions-For-Vtiger) and [An Entity Module Example](https://code.vtiger.com/vtiger/vtigercrm-manual/wikis/Developer-Docs/Develop-Extensions-For-Vtiger)
***Extension Module**
You need to re-develop the client and server code based on new [Package Structure](https://code.vtiger.com/vtiger/vtigercrm-manual/wikis/Developer-Docs/Develop-Extensions-For-Vtiger).
To get you started on it we have an [An Extension Module Example…](https://code.vtiger.com/vtiger/vtigercrm-manual/wikis/Developer-Docs/Develop-Extensions-For-Vtiger)
***Control Flow Changes**
Refer (UI Control Flow) [Internals](https://code.vtiger.com/vtiger/vtigercrm-manual/wikis/Developer-Docs/Develop-Extensions-For-Vtiger)
