Q: I am not a music person, have no interest in new Apple Music.  If I ignore this upgrade (read grab for more money) will it automatically be installed if there is a future upgrade that is really worthwhile to me?

Yes if u don't upgrade it will be installed when you do any future upadtes. But if u don't wanna use apple music apple isn't forcing you , you can ignore it and then the music app will only show just your music and the radio

You can choose to remain at whatever iOS version is currently on your device...you do NOT have to update.

However, if you ever need to restore your device for whatever reason, the newest version of iOS (currently iOS 8.4) that your device qualifies for will be installed. You can't pick & choose nor install the iOS that the device had on it before the restore.

If you ever need to wipe out the iPad to an "as new, out of the box" state, do it on the device itself under Settings/General/Reset/Erase All Content and Settings. This will clear the iPad but retain the iOS version that is currently installed on it.

There are other important security related fixes: About the security content of iOS 8.4 - Apple Support

Here are 3 of those fixes:

CoreGraphics

Impact: Opening a maliciously crafted PDF file may lead to an unexpected application termination or arbitrary code execution

coreTLS

Impact: An attacker with a privileged network position may intercept SSL/TLS connections

Safari

Impact: Visiting a maliciously crafted website may compromise user information on the filesystem

If you ignore iOS 8.4 update, you'll still be able to download/install newer iOS if your device is supported. You might want to leave WiFi switched OFF while charging your iPad. If you leave it ON, your iPad will start downloading iOS 8.4 whether you like it or not. Installation is NOT automatic. Once the update is fully downloaded, you'll be prompted to install. You may choose to ignore it if you like but iOS 8.4 update has taken X amount of space on the iPad storage.

ShagCA wrote:Once the update is fully downloaded, you'll be prompted to install. You can ignore it if you like.

You can also delete the downloaded update as well so it's not sitting on the device using your storage space. You do this under:

Settings/General/Usage/Storage/Manage Storage.where a list of apps on the device will be populated. Find the iOS Update app, tap on it and tap the the red "Delete App" button.

Be warned that it may be re-downloaded to your device in the future as well, even after you delete it. The criteria for the auto-pushed download is:

• The device is qualified for the particular iOS update
• The device is connected to a Wi-Fi router with sufficient throughput
• The device is under external power (USB port on PC or power adapter)
• The device contains enough free storage space to hold the downloaded update