 The Court has previously held that only "substantive" rule changes are considered retroactive while "procedural" changes are not.Welch argued that because the effect of the "Johnson" ruling was to change the sentence range, the law should be considered substantive (and thus retroactive).Walker, arguing against retroactivity as an "amicus", advanced the argument that, since the source of the ruling was a procedural constitutional provision, "Johnson" should not be held as retroactive.