It would be best to get the form endorsed sooner rather than later, but if you are truly unsure about when you will be moving, you can delay it for as long as you like. There is a member here who extended her process five years!!
The 6-month clock will never restart as long as you keep the vaccinations up to date AND have the microchip number printed on each vaccine certificate. Basically, the form you have from the lab in Kansas says that your dog has an acceptable level of antibodies in his/her system and if you continue the vaccination schedule (every year or every three years depending on the type of vaccine give originally) then this antibody level will always be acceptable and thats all they are really concerned about. Just to be on the safe side, you might want to contact the USDA office in your area as it is ultimately up to them if they will endorse the form.