The battery on the Librem 5 will only be replaceable “with tools,” so it won’t be convenient to swap the battery.
The good thing is that Purism uses standard Phillips screws in its Librem 13/15 and is likely to do the same in the Librem 5, so you probably don’t need any special tools to change the battery.
Given the stated goal of avoiding planned obsolescence, I’m hoping that Purism will sell at least the battery and screen, since those are the two parts that are most likely to need replacing. However, it would be even better if Purism uses standard parts found on other phone models, so that we could buy batteries and screens from multiple sources.