38 CFR § 3.951 - Preservation of disability ratings.
(a) A readjustment to the Schedule for Rating Disabilities shall not be grounds for reduction of a disability rating in effect on the date of the readjustment unless medical evidence establishes that the disability to be evaluated has actually improved.
(b) A disability which has been continuously rated at or above any evaluation of disability for 20 or more years for compensation purposes under laws administered by the Department of Veterans Affairs will not be reduced to less than such evaluation except upon a showing that such rating was based on fraud. Likewise, a rating of permanent total disability for pension purposes which has been in force for 20 or more years will not be reduced except upon a showing that the rating was based on fraud. The 20-year period will be computed from the effective date of the evaluation to the effective date of reduction of evaluation.