Chosen Solution
So this is regarding interswapping battery between units, answers says it works and people are actually confirsming this, the catch is you have to bend the flex cable slightly for shape…
That does seem to be correct, you should be able to use an iPhone 7 battery in a 6S model for higher capacity.
I have just looked at both batteries. They seem to both have the same connectors and the iPhone 7 battery is higher capacity, but strangely smaller than the iPhone 6s, so I think you could probably install the iPhone 7 battery in to the iPhone 6s, but probably not visa versa due to size constraints.
