This probably also applies to bases upgrading to 0.17.6, where this change was applied.

Any existing sniper turrets (probably also sniper gun pods etc) still have ammo in them, but don't work.

New dedicated ammo means sniper turrets all around a base and any outposts have to be unloaded and few new ammo. Which first needs to be manufactured - meaning a new build, new logistics, and potentially new train setup. Belts and inserters and logistics requests need to have non-sniper ammunition cleared away. All while your sniper turrets don't work, good luck if they were a significant part of the defense.

The recipes for sniper ammunition seem to use the same resources for the same magazine size and damage output. So it is unclear what benefits introduced by this change (if any), at the cost of forcing uninteresting and compromising base defense revisions, and no mod settings option to turn off specialised sniper ammo.

Yes, that's why I have already forewarned that in the header.

It originates from mod compatibility about sniper rifle ammo with KS Combat. That mod turns the "instant" hit ammo into a real projectile, so the ammo will hit the first entity on path instead of "instant and always hit", giving more realism. The request was to allow sniper ammo to do the same.
However, it means the sniper rifle has to use different ammo, because the "range" property has to be included in the projectile generated.

About the recipe part, real-world military sniper rifle share the same bullet with assault rifle. (Though tend to use much smaller cartridge on sniper rifle.) For example, German PSG-1 uses 7.62×51mm NATO ammo, shared with M14 rifle and M60 machine gun. So I do not feel it is realistic to intentionally make the cartridge for sniper ammo more expensive.

I am neutral to whether to apply dedicated sniper ammo or not. But I will not maintain two different versions for this mod, just for using different ammo settings in sniper rifle, sniper turret, sniper pod. (Three mods involved.)
As mod option setting is not feasible neither, since it affects multiple mods. It would be too complicated to keep from bugs. Seems here is the case that I cannot please both parties at the same time.
It is possible to revert, if majority prefers to drop the mod compatibility with KS Combat.

Doing it this way means that any reasonably sized base with sniper turrets as a major part of their defense and enjoying anticipating the UPS improvements coming from upgrading to 0.18 ... can't.

I can't even roll Factorio back to 0.17 and prepare transitional defenses without having to figure out how to install an older non-up-to-date version of a mod (if that's even possible), since this broke sniper backwards compatibility at 0.17.6. Had this only applied at 0.18 it might be a salvageable situation, but that base is basically a dead save now.

It is possible, and easy.
Just go to official download page, and download one of the "0.17.79 (alpha, stable)" there. I am not sure about MacOS or Linux ports, but on MS Win 64 port is super easy:
1/ Download 0.17.79 (alpha, stable) MS Windows (64 bit zip package).
2/ Extract to a NEW folder (not overwriting your steam version folder).
3/ Just run the bin/x64/factorio.exe directly.
4/ Game should start.
You will need to put the save files and mod files to this new Factorio 0.17.79 though. 0.17 versions of my mods are all still there. Since you are already here on mod portal, it is trivial to find and download older version 0.17.5.

Relying on auto-update (thus blind update) is a very very bad idea, putting yourself at risk.

There is always a risk with any experimental releases (currently 0.18.x).
As what Klonan said on official forum:

While we will try to avoid it, we aren't going to handicap ourselves right now in experimental by promising not to break anything.

I take the similar principle, I put breaking changes on major version, and I have tried to clarify the difference of new versions. (Otherwise I won't even bother to make the explanatory header in this Discussion section. Not much modders even bother to make such comments.)

Especially when I have changelog and announcement maintained, and I am already trying my best to perform this kind of save-breaking changes ONLY on MAJOR version. Therefore, I really cannot accept any such blame, if users ignore my effort. I have really spend LOTS of time (at least more than most other modders) to keep them well-documented.

PS: If you say 0.18 is for UPS... Nah, the devs are still experimenting EVERYTHING with it. Just like yesterday 0.18.7 update, the new poison cloud graphics (with my poison bomb/rocket) is dropping my FPS/UPS from 60/60 to 8/29... Whom should I blame? Nah, the devs are just trying to improve the graphics quality, and not expecting one to use multiple poison cloud at once. But as a result, today I will need to make an (unscheduled) update to solve this unexpected problem.
Again, 0.18 is an experimental version. If one (including me) takes it, one has to take its risk too.