Opening and old Song in a new iPhone doesn't let you open in it in the old iPhone anymore

I keep an iPhone SE 2016 (without SIM) around as a handy drum computer when the drummer of the band is not available. I can simply connect it via the phone connector to the PA. I have a simple drum track to play to.

However: When opening that drum loop in my new iPhone 12, I'm not able to open it in my old iPhone anymore because it got magically updated. That's *very* inconvenient because I have to be very careful not to open old projects in the new iPhone. And there is no way back or is it?

The new iPhone is not usable since it has no phone connector just Bluetooth. I'd have to buy a Bluetooth-to-phone-connector adapter.