SongSelect chord charts are only available as PDF files. These files are more like a photograph and contain no actual text. Because of this, OnSong cannot format songs, detect keys, and perform other basic OnSong functions on these types of files. These files are only able to be viewed "as-is".
This basically means that OnSong cannot change many things like chord diagrams and chord styles in SongSelect files because OnSong cannot recognize any text in these files. To get around this, you can open the Song Editor > tap Extract Text, and this will have OnSong submit the file to an OCR (optical character recognition) server to have a computer "read" and translate the PDF file into text. Even so, a PDF file does not contain information that is useful for aligning chords over lyrics accurately, which means adjustments are required and problems with formatting and sections will likely occur in the translated text file.
Many people have a SongSelect account which is why we continue to support it, but we also recommend services like WorshipReady.com for chord/song editing reasons. These charts are created for OnSong in text-based chord charts that work "out of the box", allowing you to utilize features like the Nashville chord system or chord diagrams.