Sprint-specific updates on Nexus 5 and other carriers
I am a Sprint customer, and am planning to switch to T-Mobile when my contract runs out in January (I know they're destined to become the same company, but T-Mobile will cost me less per month, and I'd hope to be grandfathered into that plan when they merge). I also really, really want to replace my GS3 a Nexus 5. Since T-Mobile doesn't have contracts, I would have to buy it outright then anyway, so I am thinking, "Why not just buy it now?"
My understanding is that since these devices are sold unlocked from Google Play, I can pop in a Sprint SIM now, and then when my contract runs out buy a T-Mobile SIM when I set up an account with them. But looking at the Sprint support page for the Nexus 5, it looks like they recently had a Sprint-specific update to 4.4.3 that enabled Spark (the tri-band service). I will want that update in particular while I am still with them, since I am in a city with Spark coverage.
My question is, will that update mess up my compatibility with other networks (in particular, T-Mobile)? If it does (or even if it doesn't, since I'm a bit neurotic), is there a way to get rid of it or override it entirely when I switch over? I don't mind doing a factory reset when the time comes, but in my experience those don't get rid of OS updates.
I guess I'm just confused, because I thought that the Nexus phones were supposed to be entirely independent of carriers, and only get updates from Google, yet this seems to get it's own updates from Sprint if you are their customer. Hopefully someone who knows the ins-and-outs can explain!