Quotation unit of measure cannot be changed; quotes already exist using &

What causes this issue?

The unit of measure for the quotation has been changed, but some quotes
already exist in the price quotation table (transaction O3I2) using the
old unit of measure.

System Response

The system cannot cope with quotes using the same source, type and
quote number but different units of measure: the unit of measure must
be consistent.

How to fix this error?

The unit of measure must be changed back to its original value, or all
quotes for this source, type and number must be deleted from the price
quotation table.

Error message extract from SAP system. Copyright SAP SE.