How to change file source type

Well, that's a fairly complex scenario for generally a relatively simple thing. The general advice is to use the Advanced Editor but typically when you are doing this, you are just switching from an Excel file to a SQL table for example. The basic method is that you create a new query that connects the way that you want to the new data source that you want. You then go and copy the Source and Navigation lines from that query in Advanced Editor and using Advanced Editor on the original query replace the Source and Navigation lines.

For converting from an xlsx to a Folder query, that's going to be tricky. Might be better to start with a new Folder query and use Advanced Editor to transfer over any transformation activities from your original query. Then, before you Apply or Close and Apply, rename your original query to something else, rename the Folder query to the original query name and then remove your old query. This way, when the query executes and creates the table in the data model, to the reports and such, nothing has changed.