Report group is being regenerated (change set/var. &)

What causes this issue?

The set or variable &v1& has been changed since the report group was
last generated. The current report definition and the generated Report
Writer programs thus no longer correspond with one another.

System Response

The report group is regenerated.
Caution: during the generation, the existing data extracts for the
report group are deleted for technical reasons because the data
structure of the extracts is adjusted to the old generated programs.

How to fix this error?

Recreate the necessary data extracts - if possible, in background
processing.

Error message extract from SAP system. Copyright SAP SE.