Since 1st of February 2015 Appstore ask for the 64-bit support and be built with the iOS 8 SDK. We had integrated our iOS app with unity and works fine. Currently we are using 32-bit version of unity and we use asset bundles which built from it in our iOS app. According to new regulations we have to change "Architectures" in iOS app to “Standard architectures (armv7, arm64)”. It doesn't support unity parts in the application. As a solution unity suggest to select IL2CPP for Scripting Backend which is available from Unity 4.6.2 . Is there any solution instead of upgrading Unity to latest version ?
You'll definitely need to upgrade to a Unity version that supports IL2CPP, as this is the only option to create an ARM64 build.
But you won't need the latest version (e.g. 5.x) of Unity for that, as you already pointed out, 4.6.2 would be sufficient.
But I'd recommend at least to stick to the latest 4.6.x version, as there are tons of bugfixes from 4.6.2 to that version related to IL2CPP