Substitutions of line items within a given Tier's rewards are allowed, so long as the proposed replacement is on par with the specific item it is replacing. For instance, "a plugin for the laptop" and "a partially-furnished bedroom" are two very different rewards, in both size and utility. The room could be replaced with a reward like, "a clunker of a car with wonky brakes and bad steering," but a car like this would not be suitable as a replacement for the laptop plugin. As such, while abilities could theoretically be used to replace other line items within a tier, these substitutions would have to match what they're replacing. These would not be on par with the base Tier 1 spell, as that is the biggest available reward within that tier, and has no equal within the other rewards.

For comparison, we'll say that each Tier is like a restaurant meal with sides, and drink, and so forth. At Fourth Tier 1, the room and the resurrection might each be the side courses, the unique spell the main course, and the laptop plugin the drink. While you are certainly welcome to trade your meal's french fries for a cup of fruit, you wouldn't be able to order a steak with fries and salad and have fries and salad both replaced with additional steaks.

Thank you for bringing this up! We'll be sure to clarify this rule in our next god boons update.