I also have and will continue to advocate for an option for scalling (up) a model, however.. let's make sure the right expectations are in place.
It's one thing to ask for a simple range of ring sizes.. but this gets out of hand when you're working on item such as Model Train Parts.
For reference.. HO scale is 1:87, N scale is 1:160. That means (generally) that a N scale item I design will have to be not TWO but EIGHT times more material if I request for it to be enlarged to HO scale. Hence, also eight times the cost.
The printer has a "minimum wall thickness". If you design something for the minimum wall thickness in N scale, then if you simply mathematically enlarge all the pieces, then you are making the walls 2x thicker than they "must" be.
If the designer goes back and makes thinner walls, then the true cost ends up about FOUR times as much, not EIGHT.
The one problem with this is that it prevents the designer (or a Shapeways auto-tool) from just using math to convert from N to HO. Unfortunately, to do "proper" design, they must re-design the entire item from the ground up.
The other thing here is that at the best Shapeways can produce at this time (due to that same minimum wall restriction), any detail such as a rivet must be a minimum of two inches in diameter. But, with HO, the diameter can be as little as one inch. This means that the HO models can have much finer details.. which also requires that the designer spend the extra time adding the extra details. In N scale, they're not worth it because they won't show up, but they CAN be added to HO.. it just takes more work.