Transferring apps from different apple ID's?

[amyls]

Hi, I need help. On my IPhone 5 I have an app that i have had forever, it's a free music download app that allows you to download any user uploaded content for free, and listen to it offline. This allows me to get the latest music anywhere for free. Since my phone is so old, I bought the app while my phone was still hooked up to my families Apple ID. I just bought a Macbook Air (yay), but now I have a new Apple ID that I want to use will all of my devices. Apple has made the app unavailable for purchase, Im guessing because of copyright reasons. The app still works and new music is still being uploaded, but if I use my new Apple ID with my IPhone, will I loose the app? I can't get it again, and it's saved my hundreds of dollars, so I would really like for it to be able to transfer... Any ideas? The app is called Musify. Thank you!