Type change is not possible (value '&' is not numeric)

What causes this issue?

You have changed the data type of the domain from CHAR to a numeric
data type. However, for this domain one or more fixed values are
defined which are not numeric.

System Response

The type changed is not compatible with defined fixed values and is
therefore not possible.

How to fix this error?

Correct your entry.
Before the type change the fixed values must be adjusted to the new
data type.

Error message extract from SAP system. Copyright SAP SE.