38 C.F.R. § 3.952
Ratings under the Schedule of Disability Ratings, 1925, which were the basis of compensation on April 1, 1946, are subject to modification only when a change in physical or mental condition would have required a reduction under the 1925 schedule, or an increased evaluation has been assigned under the Schedule for Rating Disabilities, 1945 (looseleaf edition), after which time all evaluations will be under the 1945 schedule (loose-leaf edition) only. Such increased evaluations must be of an other than temporary nature (due to hospitalization, surgery, etc.). When a temporary evaluation is involved, the 1925 schedule evaluation will be restored after the period of increase has elapsed unless the permanent residuals would have required reduction under that schedule, or unless an increased evaluation would be assignable under a 1945 schedule (looseleaf edition) rating. In any instance where the changed condition represents an increased degree of disability under either rating schedule but the evaluation provided by the 1945 schedule (looseleaf edition) is less than the evaluation in effect under the 1925 schedule on April 1, 1946, the 1925 schedule evaluation and award are protected.
38 C.F.R. §3.952