Alright folks listen to your pal here as I will tell you a story.
TL
R, Architectural Difference between WP7 and WP8 plus the weak hardware from WP7 prevented the upgrade possibility to WP8, however WP8 to WP9 is a possibility at least since it's the same architecture, however the "weak" hardware may prevent it from working.
You see a long time ago (3 years around) when Windows Phone 7 was still a thing. Microsoft decided to use a different type of Windows for Windows Phone 7 called Windows CE (or Windows Embedded if you prefer) now Windows CE is a kernel level program that Microsoft create for low-profile devices like smartphones, Point of Sales, lottery machines, basically anything with an embedded computer. The Windows we know is called Windows NT or New Technology (although it's been around since 2000) which is what powers Windows XP, Vista, 7, 8 and Windows Phone 8. Because of an architectural difference in the kernels it was technically impossible for Windows Phone 7 devices to upgrade to 8 because of the kernel level programming was completely different.
Of course you could simply say: Why not just remove everything from the phone including the kernel and slap Windows Phone 8 on it? Well let's be honest here, every WP8 device has at least a dual core CPU while every WP7.x device was a single core CPU, the performance issues would of been monstrous and to be frankly honest, would of made Windows Phone in an even worst position then it is now. Windows Phone is known for it's smoothness and fluid like operating system, it is not like an Android phone where it lags like a ******* when you put to much junk in it, because your hardware was too slow.
Now Windows Phone 9 I imagine will still run on the Windows NT kernel, which means technically yes it's possible, however the hardware difference from WP8 release and WP9 release may make a difference between it being possible to impossible. Of course since I said you can technically install it, some hacker would probably try it.