How to convert a song from the old format to the new one?

I'm not sure if that's the right question but here's the situation:
I downloaded a song and when I try to open it on my Mac it starts up the "old" free iReal editor. I can't seem to find a way to make it join the rest of my library in the "new" editor. It all seems to be fine on my iPhone - it's just on the Mac that this song seems to be finicky about which editor it uses. Anyone know what the trick is?! Thanks.

unless you need the old editor, delete it. It should then open in iReal Pro.
Or if you click once on the file to select it, then cmd-i (apple-i) for the info on that file, you should be able to select iReal Pro in the Open with section.
Here is an Apple support link about this:
https://support.apple.com/kb/PH14279?locale=en_US

I hope this helps.