One of the things I do with Boost dice is to award them for narrative-enhancing roleplaying in skill checks. Example: If someone says "negotiations have failed, I hit him with my club", that's just a straight attack roll. But if someone says "negotiations have failed. I turn away as if I going to walk off, keeping the club down by my side and out of sight, but then I pivot back and try to catch him with a surprise hit", then I'll let them throw a Boost die in the pool. Not just to represent the surprise nature of the attack, but also to award the player for getting into it and helping build the narrative.

So here's the change I'm considering. If the pool already has a Setback die in it for some reason, and the player does some 'narrative enhancing' to earn a Boost die, I'll give them the choice of either adding the Boost or removing a Setback. Seems like it would be roughly the same, mathematically (although I admit I'm not an expert), and it might make it easier to calculate results.

So, math experts, what are your thoughts? I'm I breaking anything? Did I already break something? Will I ever be able to play piano again?

Mathematically similar, but not the same. What you're doing is keeping the average but shifting the outliers. Removing a setback reduces the chances of large threat. Adding a boost die increases the chance of large advantage.

I think I would lean towards sticking with adding boost dice to reward a narrative because getting the outlier result is far more interesting. Then again, if the narrative is explained as removing an environmental condition, e.g.: using a tarp to protect sensitive circuits from the rain, then removing setback might make sense.

It's not game breaking, but being able to remove setback dice simply through narrative explanations does devalue those talents whose primary purpose is removing setback dice from specific checks. So there is that to consider, especially for those talents that don't do anything behind removing setback dice (Skilled Jockey and Commanding Presence come immediately to mind).

I'm not any sort of math whiz, but a boost die tends to provide Advantages to a check, as most of the facings have Advantage symbols, where a setback die as a 1 in 3 chance of adding a failure, a threat, or having no effect at all. So the boost die is more likely to either provide an extra advantage if not negate the setback die's effects, making it marginally more potent than removing the setback die entirely.