How to change GarageBand project tempo without affecting MIDI track timing?

I have a Macos GarageBand MIDI track that was recorded in free time, completely ignoring the project tempo. I want to set the project tempo to 104 (it's now 120) without changing the timing of the MIDI track. That is, this individual track has to sound exactly the same when I'm done.

There's supposed to be a checkbox for "Follow Tempo & Pitch" that I would uncheck, but it only shows on audio tracks, not MIDI tracks.

I know I could export the MIDI track to audio and import the audio, but then I would lose the ability to edit the MIDI information.

Does anyone know how to do this?