Migrating two years of history, or not

Switching analytics raises an awkward question: what happens to the years of history in the old tool?

You can import it, and it is usually worth importing. Year-on-year comparisons are the main reason anybody keeps history at all, and starting from zero in January means waiting until the following January before anything is comparable.

What you cannot do is expect the numbers to line up. The old tool counted sessions its way, filtered bots its way, and defined a unique visitor its way. Imported figures will not match the ones the new tool produces, and any import that makes them match is lying somewhere.

The workable approach is to import, label the boundary clearly, and treat comparisons that straddle it as directional. After a year the question stops coming up.