Chevron, Auer deference, rulemaking.
Where the stated intent of Congress is to implement a change, an inability to transition certain elements within the desired timeframe permits agencies to devise rules for the intersticial elements; these rules are not per se impermissible because they incorporate lapsed statutory elements of past systems.
Rules that derive from the previous statutory mandate, however, may be modified at the discretion of the agency.
A rule establishing that a value be derived from a certain other value does not require the two to be equal (or vary equally).
A rulemaking is sometimes not retroactive when the organic statute of the superseded rule has lapsed.
https://www.cadc.uscourts.gov/internet/opinions.nsf/28EC0B9175C966AF85257F2A00548088/$file/14-5330-1590892.pdf
Since 2011. An occasional commonplacing of the U.S. Circuit Courts of Appeal.
Neither an authoritative statement of the law nor legal advice. Quick work.