require the user to convert their existing code?
This depends primarily on the type of tool used to produce existing source code. A page already using valid code should not require conversion for the purpose of accessibility, although it might require the addition of information such as providing a summary for a table. A page containing large amounts of inaccessible markup from a previous authoring tool could require conversion or “repair” of the source code. Some conformant tools may make conversions automatically; while others might reject code that is invalid.
