Why would a switch in software like that that void the Warranty?
If you buy a PC, you can install any OS you like. The warranty that covers your PC is covering the hardware. If you buy a PC, have no clue what you are doing and end up trashing your windows installation, there is nothing your PC dealer's warranty will ever do for you. At least not for free. If the software is broken you reinstall it or get it serviced somewhere. If the hardware breaks down, you'll be heading up to your dealer for a warranty replacement.
Why would a phone be that much different? I even find it ashaming and harsh to realize that most people really buy that crap of "warranty is only void if you do not touch the software", like there was any warranty on the software part at all. Imagine a PC dealership trying to enforce such harsh software usability limitations like "never ever install any other software than the one you got it with, or forget the warranty". Would that actually be possible selling stuff like that? Not here in Europe at least. Imagine a car dealership that denies you your warranty on the engine after a few weeks just because you changed the seat covers. Its nothing different.
This entire "Other software voids your warranty" FUD is sparked by the providers and manufacturers that very much like to keep you trapped with them and their software, and sometimes even hold you, your device or your data hostage against yourself, pretty much neglecting the fact that you actually bought the device you are acting with, and still not wanting to give you any space to decide what you actually want to do with it. And the even worst part is, people accept it just like that.
Today's Smartphones are more like small PCs than like the old brick phones that couldnt do much. Most of these newer handsets are technically able to run many different operating systems. One can customize the systems as well, far beyond the possibilities the vendor envisioned. It sometimes feels like your PC Vendor tries telling you that you cant put any background image on your windows desktop which you did not buy from him. If you however use your own images, or god beware, remove the logo of said Vendor from the starting screen of the OS that that would be a change that possibly damaged your hardware which in turn would be void then..
Think about it.