Nope, with Windows RT you could neither side-load unsigned Win32 applications, could not develop native Win32 applications nor could you emulate existing x86 applications.
All these things are essentially gone.
With Windows 10 for ARM you are getting a fully native ARM 64-bit Windows OS with the capability to not only side-load and run all native ARM64 (both UWP and Win32) programs but due to 2 WoW layers also all ARM and x86 32 bit programs (both UWP and Win32).
In addition all tools like Visual Studio and the Windows SDK are in place to develop or re-compile native 64 bit ARM apps (both Win32 and UWP).
Regarding the upgrade question, that depends on your use-case. If you mostly need to run apps, which only have x86 or x64 versions then you might not want to upgrade. However if you mostly running native ARM64 apps and some x86 apps on occasion, the upgrade is most likely excellent.