Thank you for the additional information, although I don't see where you addressed what duties you no longer perform and why. That is very important in regards ***** ***** issue. If you cannot perform some of the "essential" duties of your job, with or without accommodations, then you may not any longer be filling the position description you were filling before your MS symptoms began. In that case, your position description could be changed to coincide with what you can actually do, which could result in a downgrade of the GS level, or you may be placed in a completely different position that you can perform all of the essential functions, but could conceivably be at a lower GS level.
I can't guess what your specific situation warrants or what success you might have challenging the downgrade, but, as mentioned above, it can legally happen in some cases. It would be unlawful if you were targeted for an adverse employment decision simply because of your disability, but that is different than having your position and paygrade adjusted for the duties you can actually perform.
I would suggest that you file a disability discrimination complaint with your EO Office and let them investigate the situation. If, after they do so, you don't feel comfortable with the conclusion and resolution they reached, then you might consider sitting down with a local employment law attorney who handles Federal Government employee discrimination cases. Once they can conduct an in depth interview with you and perhaps gather some additional information from the employer, they can give you a better idea if they believe the actions taken by your employer are in violation of either the ADA or Civilian personnel procedures.
Please feel free to ask for clarification if needed. If none is needed, then if you could take a moment to leave a positive rating in the ratings box above, I will receive credit for assisting you today. Thank you