Keeping a year-on-year chart honest after the app changes address

Finance will still want last March next to this March. After a platform move that is a request to compare two different instruments. The honest version of the chart has a seam, a date, and a sentence about what changed.

We ask teams to pick a freeze Sunday and to treat the following four weeks as a parallel-run if the old app is still in the wild. During that window, both properties are recording. The destination figures will look low; that is expected while the old version still accounts for most sessions. The useful artefact is not a single line. It is a small table: old property, new property, overlap, and a note on any event that exists in only one column.

Year-on-year becomes defensible again once three things match: the identifier that defines a user, the event that defines the action, and the clock used to bucket the day. Miss any one of those and you are looking at a new series that happens to share a name with the old one.

If the destination vendor forces new event names, keep a mapping sheet in the same folder as the board pack. When someone later asks why ‘checkout_complete’ in 2025 is not ‘OrderConfirmV3’ from 2024, the sheet is the answer. Without it, people redraw the chart until it looks smooth, which is how a move gets remembered as a traffic miracle.