FYI, PES is http://en.wikipedia.org/wiki/PULHHEEMS.
1) Go to Govt/Private Specialist.
2) Consult specialist, and do whatever medical check ups required. E.g. physical exam, blood test, x-ray, CT scan, MRI scan.
3) Specialist gave a finalised clinical diagnosis.
4) Get the specialist to write you a memo which is to be given to SAF MO. Those who can and are willing to spend some more money can get him/her to write you a specialist report, which is more precise and detailed.
Note: For not to waste time and money, please be more direct yet humble. Request the specialist to write about reviewing of PES and medical board / anything specific such as any excuses to your conditions.
5) Book an medical review appointment using the eHealth module @ www.ns.sg (for NSman Reservists).
Just go down to your camp medical centre (for NSFs).
Call CMPB @ 6373 1340 to request another PES review (for Pre-Enlistees after CMPB Checkup).
6) Make a trip down to the respective unit camp medical centre. Give the MO whatever supporting documents you have in hand. E.g. Specialist memo/report (most usually the case and highly recommended), x-ray films, CT/MRI scan report, blood investigation lab report... etc.
7) MO will decide whether you are deemed eligible for KIV downgrade, according to the criterias set in the "PES Bible" directive. E.g. Diagnosis, Extent of Diagnosis, Degree of Diagnosis/Injury/Illness/Disease...etc.
8) If deemed so, you will sign an undertaking note of medical board, whereby your case statement will be prepared for discussion at the monthly medical board meet (usually at the Formation/Division HQ) with another NSF CPT MO, and the Chairman (SAF Regular Medical Doctor of MAJ rank and above). Your Medical Board result will be post mailed to you by your respective NSHRC (Formation NS Hub).

Time and time again, the questions for downgrade is repetitive. So this serves as a general SOP for Medical Board.

Extracted from here:
NSmen must update their NS HRCs if they should develop any new medical condition or if their existing medical condition has worsened which might affect their ability to perform their duties during ICT. Arrangements will then be made for them to attend a medical review at the SAF medical centre to assess their fitness for NS.

The NSmen MUST bring along all investigation results and memorandums from his external physician/specialist during the medical review. Depending on the outcome of the medical review, the NSman may be given a medical certificate to defer him from ICT, be scheduled for a medical board to downgrade him if his medical condition is significant, or referred to a restructured hospital for further investigation. In the event that the medical officer determines that the NSman’s medical condition will not affect his ability to participate in the ICT, he will allow the NSman to attend the ICT.