Instructor certifications are good for a period of 4 years. Existing instructors can take updates multiple times and sucesssfully complete their update at any point during that time. The normal outcome is a clean pass - their skills remain current and they demonstrate proper teaching skills. In some cases, an instructor may choose to complete an update at a lower level (an IT/ITE at their level might not be available, they no longer teach classes at their highest level, etc.). An unsuccessful completion is also possible, perhaps their skills are no longer at their current level. You can also help them with some sort of corrective training - very similar to continuing a new instructor.
Now for a dicey matter, what if the instructor candidate is totally unsuitable. Perhaps their skills have degraded considerably or worse yet, clearly demonstrate a disregard for student safety. You will of course mark them as unsuccessfully completing this update and give them a thoughtful explanation/debrief. You should also contact the SEI office and request a formal challenge. This challenge process is well documented in the SEIC Policy manual in Chapter 5, Section C.