...It may not be possible. If you haven't seen a link via c0deking or Wind, is probably isn't possible.
The reason you can crossflash a 1633S (and I'm just basing this on what you've said here, I don't own the drive so I haven't looked in to it specifically) to a 1653S is because those two models are mechanically identical. If you haven't seen (here, at c0deking's site, or similar) a crossflash firmware from 1633S to 1673S, DO NOT attempt to crossflash, as this will more than likely make the drive unusable/unrecognizeable by the system. Also keep in mind that crossflashing will void any warrenty you may have on the drive.