Headstogether
There is no official limit, providing you can show that the evidence is still relevant to your conditions as they are now then it should still be acceptable, however, those assessing will prefer information that is less than two years old and the new the better.
It will be an assessment for ESA, nobody at this time is being moved to UC unless they have to claim one of the other benefits that fall under the UC umbrella.
Gordon
Nothing on this board constitutes legal advice - always consult a professional about specific problems 
