Chin Implant Revision - Nearly 8 weeks ago, I had a chin implant. It is too large (1cm) and created a "witch's chin" profile.

Revision chin implant surgery can be notoriously difficult.  Make sure your surgeon determines that you don't have any complications, other than an implant that's too large.  One option is to remove the implant and have the implant pocket close in, then proceed with revision surgery.  Unfortunately the implant pocket does not always shrinks down, and can leave the patient with a droopy chin.  In my practice the preferred option is to proceed with revision surgery using the correct size special implants that tend to adhere to the tissue.  This approach is more predictable in avoiding the undesirable results that can be seen in revision chin surgery.
 

I would return to your surgeon for post-operative evaluation, to rule out any other causes for experiencing this type of result.  Based on his/her findings and your unique case, your surgeon can deduce when it would be most appropriate to move forward with a revision surgery.

From the photo you submitted, your chin does look too big in my opinion.  You need to return to your surgeon for an exam, and assuming any other causes are ruled out, there may be an argument for reducing the size of the implant sooner rather than later.

it is difficult to answer this without examining you but assuming that all is healing well and there is no infection or hematoma, 2 months is an OK time to revise or replace the implant

First and foremost, you should schedule an appointment with your surgeon so that he or she may examine you to make sure that there is not a hematoma (blood pocket) surrounding the implant or that there is not an infection.
If you desire a smaller implant, the current one needs to be removed and your chin needs a chance to heal before replacing it. I would recommend waiting 3 to 6 months.