I am not so sure, I would be messing around with a FDC as I would not want to chance having it convert to a standard claim.
I did find this one reference http://www.benefits.va.gov/fdc/walkthrough.asp and scroll down to the verification section where you are certifying that you are sending in no additional evidence. Now whether the VA will consider sending in any evidence, even for a new claim as grounds for conversion, I'm not sure. But knowing the VA, I would not want to take that chance.
When a veteran is 100% from a original disability and files another S.C. disability that is rated at least a 60% rating then VA has the duty to assist with a statutory rating for SMC.
Some times they don't do that but hopefully the veteran will catch it.
SMC pays an extra 347 monthly with spouse.