The High Aldwin
Maybe I missed something in the discussion, but what "rule" are you referring to?I respectfully challenge your notion, but I am not trying to convince you to like the rule. The effects can be rationalized and abstracted to agree with both the physical and magical nature of the fantasy world. For example, why can't a ray of fire slow you down as you hobble from the burns you just suffered (because that is how the spell works...to slow you down)? Chain fire...it's magic...why can't fire jump--fire elementals can? I argue this points because I was starting out with the same "how to I explain" perspective, and I realized that there must be a more rules-specific reason as to why making one type of damage would be a problem.
I posted above I would allow a caster to learn a different damage type (especially elemental damages) for a spell, but it would be a separate spell from the original. So, if a PC learned fireball, they could learn a separate spell lightning ball, simply substituting lightning damage for the fire damage.
Now, knowing one spell and being able to change the damage type when the spell is prepared is a bit more extreme, and changing the damage type when it is cast is really too much IMO. Prepared is feasible (when studying the spell you alter the formula slightly to change the damage type), but spontaneous when cast should have a cost: a feat, very limited uses per day, metamagic (there was a UA one that did this IIRC--it might have even made it into Tasha's but I can't recall...).