Is it at all possible to Migrate Matrix and Wygwam fields to native Grid and Rich Text fields in version 6?
You could create an XML template that outputs your wygwam and matrix field data and use Datagrab or Smart Import Export to import it to the new native fields.
Wygwam fields should be able to switch those without issues. Matrix stores it's data differently, so exporting / importing should be an option. In V2 there was a script (somewhere) that provided a migrration from matrix to grid.
here's the Playa & Matrix Importer script Jelle is talking about https://expressionengine.com/add-ons/playa-matrix-importer