Attachments and .html files in Classic Knowledge are moved to the Files object, Visualforce pages refer to Classic article types, and article numbers change during migration are statements that should be considered when migrating an existing knowledge base into Salesforce Lightning Knowledge. These statements reflect some of the changes and challenges that may occur during the migration process. For example:
* Attachments and .html files in Classic Knowledge are moved to the Files object because Lightning Knowledge does not support attachments on articles. Files can be related to articles using rich text fields or file fields.
* Visualforce pages refer to Classic article types because Lightning Knowledge does not support Visualforce pages on articles. Visualforce pages need to be replaced with Lightning components or other solutions.
* Article numbers change during migration because Lightning Knowledge uses a different numbering system than Classic Knowledge. Article numbers are prefixed with KA in Lightning Knowledge instead of KB in Classic Knowledge.
Verified References: Service Cloud Consultant Certification Guide & Tips, Migrate Your Salesforce Classic Knowledge Base to Lightning Knowledge