Yeah, I was going to say that. There needs to be hardware capability, and most of the phones already on the market don't have it.
It's more than just an OS upgrade the way I understand it. While this is something I'd like to have, I'm more eager for Continuum, which also requires new hardware, and which I would very much like to invest in.
While your current hardware may not support USB-OTG, it will support Bluetooth mouse and keyboard. WP8.1 Update 2 supports Bluetooth keyboard, which can be nice when you're doing the "Project My Screen App". Using a Bluetooth keyboard frees up more screen area, too, because it doesn't display the on-screen keyboard when its connected. Still not Continuum, but makes for easier editing/adding contacts, doing email/texting, and other things... IF you're at the office or at a desk with a computer.
Basically all USB-OTG is going to give you is the added support for flash drive storage for backing up your data or easier sharing of data with others.