iPhone 6(S)/Plus Battery replacement on old iPhone running old iOS

[now i see it]

So I've got an old iPhone running iOS 9.3.5 on the original battery and the battery has seen better days. A battery replacement is on the horizon.

Since I live a million miles away from an Apple Store, I'll have to mail my iPhone to them (I have a backup newer device). Before I do that though I'll need to back it up to iCloud and iTunes and restore it (wipe it) as new before sending it to apple

Question 1:
Since iOS 9.3.5 obviously isn't being signed by Apple anymore, can a virgin fresh copy of iOS 9.3,5 be installed on my iPhone during a nuke restore before sending it in since I already have it installed?

I know I can restore from my iOS 9.3.5 iCloud or iTunes backup to iOS 9.3.5, but that won't accomplish anything cuz I'll be exactly where I started with all my data & apps back on the phone.

If I nuke & restore my iPhone to iOS 12 before sending it into apple, I'll never be able to reinstall my old backup again.

Question 2:
If I want a new battery installed by Apple, will my iPhone have to run iOS 12 from then on?

(or maybe it's time to upgrade the OS. Talking about an iPhone 6 Plus)

 

[BugeyeSTI]

You can try to use "erase all contents and settings" instead of doing a restore without backup as I always was under the assumption that it would leave the version of iOS on the device had on it. Some had said that this isn't true anymore and you'll be forced to update to the latest version of iOS that your phone will run which in your case will be 12.4.4. Since Apple doesn't sign 9.3.5 for your phone anymore, a fresh copy is impossible. If you really want to keep 9.3.5 you can either try my first suggestion or you could order a battery replacement from ifixit and replace the battery yourself. If you're set on sending it to Apple, just restore to iOS 12.4.4 as it was a great update for my backup 6S and fixed a lot of the lag people complained about on older phones like your 6+.

 

[eyoungren]

So I've got an old iPhone running iOS 9.3.5 on the original battery and the battery has seen better days. A battery replacement is on the horizon.

Since I live a million miles away from an Apple Store, I'll have to mail my iPhone to them (I have a backup newer device). Before I do that though I'll need to back it up to iCloud and iTunes and restore it (wipe it) as new before sending it to apple

Question 1:
Since iOS 9.3.5 obviously isn't being signed by Apple anymore, can a virgin fresh copy of iOS 9.3,5 be installed on my iPhone during a nuke restore before sending it in since I already have it installed?

I know I can restore from my iOS 9.3.5 iCloud or iTunes backup to iOS 9.3.5, but that won't accomplish anything cuz I'll be exactly where I started with all my data & apps back on the phone.

If I nuke & restore my iPhone to iOS 12 before sending it into apple, I'll never be able to reinstall my old backup again.

Question 2:
If I want a new battery installed by Apple, will my iPhone have to run iOS 12 from then on?

(or maybe it's time to upgrade the OS. Talking about an iPhone 6 Plus)

Question 1, already answered by @BugeyeSTI, although I'll just say that Apple isn't allowing activation of OS9 on certain iPhone models (mainly the 6s/6s+). So, even if you could restore to it, you'd possibly be forced to update anyway.

Question 2: If Apple does not update your iPhone then no, you won't be forced to run iOS 12 (or iOS 13).

However, you can expect Apple to update your phone when you send it to them. It's one of the first things they will do. You can ask that they do not, but don't expect them to honor the request.

 

[now i see it]

Thanks for that info!