Hi Giancarlo,
No the old phone load will still exist, just make sure that you take note of the settings under 'Device', 'Device Settings', 'Device Defaults'.
This tells the phones what file they should use when they boot up. If you install the new fireware file it will update this setting, meaning phones will start to use the new firmware automatically after a reboot!
If you want to use the old firmware simply change the setting back to the old phone load name after installing the upgrade. You can then change the phone load for a specific phone on it's device page and test if it solves your issue before upgrading all your phones!
Hope that helps
Matty