The reason the tread is gone is because the material has worn away. Soldering in new tread means melting more material so that the tread is closer to the foot and has less cushioning. If you have another set of crocs, or even interlocking floormat foam you can instead try the resole option using contact cement. This will add material that will cushion the foot while giving more material to melt in the new tread. Results may vary so look up a good tutorial on what adhesive to use. You want something that won't have residual stickness and high adhesive grip properties to keep the new sole in place without getting goo in the carpet.
This method isn't designed to get you back to 'good as new'. It's more like 'get you by for a while longer'. To me, Crocs are not exactly unique or expensive enough to bother with re-soling. I think I'd just buy a new pair.
