My device says I have to transfer my Apple ID to get Apple Music and ibwont be able to change it back for 90 days, what does this mean?

It means you will be starting a 90 day association to that account, so you won’t be able to specific things (e.g. redownloading last purchases, automatic downloads etc.) with a different account until those 90 days have completed : View and remove your associated devices in iTunes - Apple Support

If you have multiple accounts you could put them into a Family Share