In my opinion, the two logical files do not change due to this modification. Appearantly, the data-element that will be part of the primary key of the second LF after the change already existed prior to the change. The only thing that changes in the LF’s is that this particular data-element will be part of the primary key. Looking at the counting guidelines, this is no reason to identify a changed LF.
I agree with Martin that a constraint on the LF changes, but that will be expressed in changed user transactions that use and/or modify that particular LF.