I was actually thinking of getting a new snake soon and possibly housing it in the 3x2x2 enclosure while my current BP gets upgraded to a 4x2x2, so you read my mind there haha. I haven't 100% made up my mind on getting the new snake right now, but I'm thinking down the road I will probably want to upgrade my boy to a 4 foot PVC anyways so the 3x2 would need to either be used by another juvenile BP once I decide to get one or I'd have to sell it.
Another argument I've heard for 4x2x2 enclosures is that they're the best size for establishing a proper heat gradient, and that smaller enclosures don't give enough space for the snake to effectively thermo-regulate. I don't know how accurate this is, but it seems like every keeper that houses their BPs in a 4x2 says their BP uses all of the space and becomes a lot more active in general - which is why I'm leaning towards eventually upgrading even if he doesn't technically "grow out" of the 3x2.