You are changing material group, but material already used by purchasing

How to fix this error?

When changing the material group of a material that is already being
used by the purchasing department, you should note that:
The purchasing statistics show how much of a material group has already
been ordered. Unitl the change is made, the material is managed under
the old material group and therefore the old material group statistics
are updated. From the moment the change is made, the new material
group's statistics are updated.
Some purchase orders may derive from contracts with item categories
referring to material groups. If this is the case, the material cannot
be used after the change has been made.
Every purchase requisition is allocated to a release strategy that can
depend on the material group. A new release strategy may apply to a new
material group. Purchase requisitions that were created before the
change are subject to the old strategy; purchase requisitions that were
created after the change are subject to the new strategy.

Error message extract from SAP system. Copyright SAP SE.