I don't see any explicit reason this isn't allowed, it might even be a bug? But it feels weird coming from other mods to not be able to easily replace sections of my cable network, instead needing to tear down the whole thing and replace it with the more expensive cable all at once. It would be nice if I could just mix and match tiers of cable instead.

I'm not sure what you mean by "transformers"? This isn't Immersive Engineering.

Use transformers for that purpose, e.g. running HV throughout your base and turning it into MV for your many assemblers or low power machines in one area

oh nvm i thought this was another mod lol, sorry about that

ik I'm late but you can always use a starter energy cube or something to separate the cable tiers

Connected cables of the same tier act as a single cable network, so mixing them might cause slightly worse performance. But all the code should still work fine, and the "not be able to easily replace sections of my cable network" argument is perfectly valid and I think sufficient.