When should I change my iPhone?

My battery health is art 96 percent after 23 months. The battery is still ok.

That's still highly acceptable.

I got Apple to replace the battery on my four year old iPhone 6 128GB last month: battery efficiency had fallen to about 86% (IIRC) with a noticeable drop in duration but until about 3 months before that the battery duration was good enough that I hadn't noticed much loss of performance.

But I took that change as a prompt to upgrade to an 8 Plus as I needed the larger screen, additional storage and water resistance due to a change in my job. The old phone is still working just like new with the replacement battery and my dad now has it.

If your current iPhone does what you need still then it has plenty of useful life remaining and is probably now fully paid for.

When you're dying for a bigger, more expensive phone. I still have my iphone 5, and I'm perfectly happy with it.

It is OK for now.

Don't waste thought on it until a problem develops.

The battery is happiest at less than full, and not dead. You're doing OK as long as the power isn't suddenly dropping when you turn it on. That would be an old, damaged battery, failing from age.

Whenever you feel like upgrading.

There's no mandate that you should, like in this case if you are going on the battery's health. I personally change every 2 years. Some change when their iPhone is no longer getting iOS updates.

Again, entirely up to you.

Every 2 years that's when battery life starts going down