Root, that's what I always thought. But here is a quick find:
"Subsequent Legislation
If the views of a later Congress are expressed in a duly enacted statute, then the views embodied in that statute must be interpreted and applied. Occasionally a later enactment declares congressional intent about interpretation of an earlier enactment rather than directly amending or clarifying the earlier law. Such action can be given prospective effect because, “however inartistic, it ... stands on its own feet as a valid enactment.”332 “Subsequent legislation declaring the intent of an earlier statute is entitled to great weight in statutory construction.”333
332 F. REED DICKERSON, THE INTERPRETATION AND APPLICATION OF STATUTES 179 (1975).
333 Red Lion Broadcasting Co. v. FCC, 395 U.S. 367, 380-81 (1969). By contrast, a “mere statement in a conference report ... as to what the Committee believes an earlier statute meant is obviously less weighty” because Congress has not “proceeded formally through the legislative process.” South Carolina v. Regan, 465 U.S. 367, 379 n.17 (1984)."
I guess we can debate whether or not a yet undecided case on a not yet interpreted law is prospective or not. But, the court doesn't even need to go that far; the court can simply rule without explaining, or it could rule by adopting MLB's interpretation without even referencing the new amendment.