Dancing between light and shadows

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For the houserule question, skip down passed TLDR.


So I'm currently building a Blightspeaker Invoker with the focus being a mix of radiant and necrotic energies. A soul who must walk the middle ground and choose who lives and who dies.
I'll be taking some healing via multi-classing and I've set up the majority of my powers with fear/death themes. Radiant damage is all fine and good, but sometimes you want a dark themed attack to show that something else is at work now.

TLDR
The Blightspeaker can switch to necrotic power by spending an action point.
The DMG suggests switching radiant keywords for necrotic on divine characters serving evil deities.

The question is:
What would it take to reasonably switch a radiant power to necrotic at least once per encounter?
How would this be best balanced?
What should I give up for this option?