Swapping (permanently, not on a need-basis during the game) a spell's damage type does not alter the game balance, as long as the damage type is in the same category of usefulness (this may sound a bit vague, but the game designers have given hints based on how many monsters have immunity/resistance to various damage types, that at least you should not swap elemental damage with force or radiance because there are few monsters in the game that can resist these two damage types).
I did some analysis of damage types here. Summary: Poison is by far the weakest; then fire, lightning, and cold are clustered together; then acid and necrotic; and the other damage types are almost unresisted.
On the subject of Potent Cantrip: Mechanically, it got a lot better in Xanathar's, since the most powerful damaging cantrip is now toll the dead (decent range, d12 damage in most cases and falls back to a still-respectable d8, Wisdom save, necrotic damage). So it's now a pretty useful ability. But flavor-wise, it's kind of a fail since toll the dead is necromancy. The go-to evocation cantrip is fire bolt, which doesn't benefit.
Potent Cantrip is roughly equivalent to +3 on spell attack rolls. (Assuming targets have a 70% chance to fail their save, half damage on a successful save means the cantrip averages 85% of its base damage. 70% to 85% is what you get from a +3 on an attack roll.) So I'd consider rewriting it like this:
Potent Cantrip
Starting at 6th level, your evocation cantrips affect even creatures that avoid the brunt of the effect. When a creature succeeds on a saving throw against your evocation cantrip, the creature takes half the cantrip’s damage (if any) but suffers no additional effect from the cantrip. In addition, you get +3 on spell attack rolls with evocation cantrips.
So, rather than pushing you toward cantrips which grant saves, it now pushes you toward cantrips in your school.
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