It all depends on the connection between the existing flange and the soil pipe, and the vertical dimension of the offset flange you use. If the current flange is tightly-glued, you'll need to remove a bit more of the soil pipe and then add a short run and coupling to bring things up to the new flange's bottom. Hopefully you have easy access (as in a basement, or deep crawl space).
I installed a special-order Kohler Wellworth a few years ago, with the 10" flange-to-wall distance. Turns out I could have used the standard 12", as the Wellworth's tank is about 2-1/4" from the wall. The thing plugs up every now and then, unlike the American Standard Champion 4 we installed at the same time in the upstairs bathroom--that thing doesn't know what a plunger is!