1. Prius Prime came with a redesign. Essentially Toyota redesigned the car, and one of the major changes in addition to new look is that trunk bottom could be lifted up to fit much larger battery for PHEV variant. Before that, this wasn't possible.
Same thing happened a couple of years later with CH-R. Redesign, car gets a completely new look, and one of the new design changes is a possibility to dedicate some of the trunk space to extend the battery.
So no, you cannot just grab an old HEV and slap a bigger battery into it. You need to redesign the car for this to be possible, at least on industrial scale. I'm sure there's some small shop doing mods even to existing cars.
2. As I referenced above, it is NOT in fact a solved problem, as demonstrated by the fact that current Corollas struggle to fit even a mildly upscaled power pack of the 2,0L variant, having to move 12V battery away from engine bay to make it fit. Whereas 1,8L variant with about 1/3 weaker MGs has enough space to fit 12V battery under the hood.
Essentially redesign is needed to make this work for most cars where space under the hood is already pretty tight.
And again, this is exactly what we're seeing Toyota do. PHEV variants become available with new look and redesign of each model, at least for more compact cars. First Prius, then CH-R. RAV4 may be able to take the bigger battery with less effort, since car itself is so much larger that others.