From an Occam's Razor POV: it would be simpler for some agency to require a backdoor or transfer the character-based unlock, which they could apply whether the phone has a finger-reader or not. Since I haven't read the entire codebase of any of the operating systems I use, my paranoid self says "Expect root-ation!" while my normal self says "Get back to work, quit reading and writing internet commments!"

Given the fact that we've seen no evidence of such updates previously, and rather we've seen Apple take many steps to protect privacy and even seen NSA leaked slides detailing how they need access to PCs to read iPhone backups, I think it's extremely unlikely that they would ever take the risk of pushing an customer-wide software update.

If you are worried about being specifically targeted, you shouldn't use an off the shelf cellphone at all, and this discussion is moot.

Is it possible for software to even read the fingerprint from the sensor, as opposed to only asking the sensor if this fingerprint matches a previous fingerprint????

How about hardware design doesn't allow it because the thing being stored is a hash?