Your chin reduction is reversible. It could be done by either undoing the sliding genioplasty or putting in a chin implant for augmentation.It would make the most sense to just undo the osteotomy and put the bone back in its original place since the problem you now have is that the vertical dimension of the chin is too long. Your case illustrates that a setback sliding genioplasty is not an effective chin reduction procedure and increased vertical length is the trade-off for a backward horizontal movement.
Yes, this can be reversed. It appears the chin has been inadvertantly elongated as well as set back. Through thee same incision, the plate can be removed and the chin repositioned.
A sliding genioplasty is possible, but more information is required to fully evaluate if a secondary, revision surgery is possible since bone was removed and scarring plays a factor. We are not normally partial to using chin implants to increase projection. We cannot guarantee 100% that the chin would be in exactly the same position as before although this would be the aim.
You had a nice chin before. I agree with you that the chin is too flat and long at this point. Sliding genioplasty could correct this.