infantry36,
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Generally, you have to show that the condition was unfitting when you were placed on the TDRL.
AR 635-40, Para. 7-20,b., :
"b. Changes in a Soldier’s condition while on the temporary disability retired list. The combined percentage rating approved at the time the Soldier was placed on the TDRL cannot be changed by the PEB throughout the period the Soldier is on the TDRL. Adjustment will be made at the time of removal from the TDRL to reflect the degree of severity of those conditions rated at the time of placement on the TDRL and any ratable conditions identified since placement on the TDRL. An EPTS factor may be added, modified, or eliminated at this time if additional evidence is obtained that was not previously available or apparent during the initial evaluation; or placement on the TDRL was due to fraud, mistake of law, or mathematical miscalculation.
DoD Instruction 1332.38 contains more specific guidance on this point.