23, 104 S.Ct. 296, 78 L.Ed.2d 17(1983))); Black, 11 Vet.App. at 17. The M21-1 modification similarly reflects the Secretary’s ability to define “systemic” in a limited way when he wants to do so, but such a modification cannot serve to retroactively modify the plain meaning of a regulation. See Bowen v. Georgetown Univ. Hosp., 488 U.S. 204, 208, 109 S.Ct. 468, 102 L.Ed.2d 493 (1988) (“Retroactivity is not favored in the law. Thus, congressional enactments and administrative rules will not be construed to have retroactive effect unless their language requires this result.”). Indeed, a change in the M21-1 has no effect on the plain meaning of a regulation, which can only be modified after VA fulfills the notice and comment requirements of 5 U.S.C. § 553. See Theiss, 18 Vet.App. at 213 (<HOLDING>); see Smith v. Shinseki, 647 F.3d 1380, 1385
holding that substantive changes made by administrative agencies in regulations are required to comply with certain notice and comment requirements which include publication of a notice of proposed rulemaking in the federal register an opportunity for interested persons to comment on that notice and after consideration of these comments publication of the final rule with a general statement of its basis and purpose citing 5 usc  553b c holding that requiring notice and comment is not required where the record demonstrates that the agency in fact has had the benefit of petitioners comments holding that form 4790 is a rule that requires notice and comment prior to adoption and publication in the federal register before it can support a forfeiture action based on an individuals failure to fill out the form holding that the postal service was not required to follow notice and comment rulemaking procedures holding that governments choice to publish notice in the new york times did not violate statutory publication requirements even though publication in the houston chronicle would have been more likely to provide notice to the claimant