We recommend upgrading to the latest available version—i.e., from Kentico 11 Portal Engine to Kentico 12 Portal Engine. MVC is the recommended approach and, therefore, customers should consider transitioning as soon as possible.
1. If a site is currently at the end of its lifecycle and is about to be redesigned, the best approach is to create the new one based on MVC. During the redesign, most likely, some content will be reused, and so will OM and EC data, and some extensions, etc. The front end will probably be created from scratch.
2. If a site will come to the end of its lifecycle in the following years (by 2021), the best approach may be to wait and redesign at that point.
3. If the site is not going to come to the end of its lifecycle in the near feature and full support will still be required, the transition from Portal Engine to MVC must still be performed.
There is no tool to automatically convert Portal Engine front end to MVC front end because the two technologies are very different at an elementary level. It is subject to being validated on whether we can find a (partial) solution for certain customers.
Structured data independent of the development model (e.g., system configuration, online marketing, commerce data, ...) are upgraded automatically. Content created and stored on a Portal Engine based site (e.g., content stored in widgets/webparts, Portal Engine specific page types) won't be upgraded and need to be migrated.