Conversion of SharePoint Classic Webparts or Addins to Modern UI

SharePoint Modern UI offers significant technical improvements and better User Experience to all people, using SharePoint for Collaboration and as Information Portals. Microsoft encourages users and Administrators to use Modern UI by default for the newly created Site Collections, especially in SharePoint Online.

Unfortunately there is one bad surprise, awaiting those, decided to switch from Classic UI – and it is a backwards incompatibility of Webparts or Addins with new Modern UI environments. An obvious question – how serious is that? How difficult might the migration be?

I have collected a rough minimal list of changes to be performed to migrate existing classic solution to new SharePoint Framework below:

1. Create new SPFX Solution with modern Microsoft tools
2. Migrate UI templates from “*.ascx”-Format into React (if necessary)
3. Switch Data layer access code from SSOM/CSOM/JSOM to REST API Queries.
4. Convert CSS, update or remove references to classic SharePoint styles from templates
5. Change the deployment model

As we see, almost every single part of the Classic solution must be updated, or possibly even completely rewritten to comply with Modern UI. One might say, that migration of Classic solution will take no less effort, than development of a new solution with the same business functionality. I am not that pessimistic, but based upon previous experience, I would estimate the time, necessary for migration to 50-80% of the original effort. In most applications it means virtually throwing existing code off and building up a new solution.

As a conclusion, I could only suggest to think twice before considering a migration of classic SharePoint solutions to Modern UI and plan enough time and effort for this.