In my opinion, the IFPUG's enhancement equation may not be used in isolation and should always be used as a sequel in adjusting the Application FP count where the Change Before and Change After are accounted for. Even a fractional change in an entity will yield as many FPs as when counted as newly developed.
NESMA seems to be the best Functional Size Measurement when it is to be applied to purely size the enhancement projects.
Request wider forum's views.
Thanks & Regards,