During assembly the dimension of a particular beam don't matter that much. The fact that this beam has to moved through an assembly line does. Once you remove that beam assembly is getting a lot easier.
You are right regarding swapping a fuel tank for a battery. But that was before they went to re-tooling assembly lines for EV vehicles. So it is easoer to run both cars through the same, specialized line. But that is the whole point, isn't it? Once these manufacturers did the re-tooling that doesn't matter anymore. And as soon as number go up even existing plattform are not that important as the are now.
You're right, I'm not an ME. I'm a controls engineer...and it's the same software, controls, and 90% of the machine that run your different tools for different years.
But the quoted sentence is the important one. You can change the shape of a beam...but that beam is there in your process because you assume that there's no structural battery pack to take the load instead, or that there is a huge, heavy, rigid engine block under the hood that needs to be accomodated. If you changes those assumptions you would need more than new tools to go on the current presses and robots and conveyors, you'd need a whole new process.
I have installed brand new machines less than a mile south of Greenfield Village at the Ford Product Development Center, where they're willing to spend millions on new equipment to develop new products but not really thinking about building new processes.
Changing the form of engine mounts or getting rid of them all together is in effect the same thing and purely driven by design. In fact, getting rid of engine mounts makes production a lot faster and cheaper. So yeah, retooling in that regard means re-tooling. They will loose some benfits from their plattform strategies, but that is the main reason most EVs from traditional car makers are, as of now, based on existing plattform. Given a positive business case new plattforms will be developed soon enough, if need be. And EVs can still benefit from other common parts.
EDIT: The exption to the plattform usage is BMW, they went the opposite way. Also to develop knowledge on the use of composite materials that can be used in oter model lines. So yes, there are some generalizations in my post one can find counter examples to.