I am currently researching this as we will soon be advertising an FTRS post. The definitive answer from the FTRS folks at Air Cmd is that if you are on AFPS 75 there is no requirement to take a 30 day break (you do have to retire from the Regulars and become commissioned in the Reserve), but if you are on AFPS05 you need to have 'retired' for a period of 30 days.