Every cohort and conversion report that spans the change becomes a comparison between two different funnels. Map old stages to new explicitly, keep the historical stamps intact, and annotate the change on the dashboards — a silent stage edit is one of the classic ways executive reporting quietly stops being true.
Treat stage changes as a versioned event with a date and an owner. Keep a mapping table so prior-period numbers can be restated on request, and decide up front whether you restate or show a break in the series — either is defensible; switching between them per deck is not.
Every claim above comes from work LeanScale published. These are the sources.