Diagnosis
You want to use mass change to replace one material in BOMs withanother. This is only possible if the old and new material have thesame base unit of measure.
The same applies to the issue units for the two materials.

Procedure
You have two options:
Enter another material which has the same base unit of measure as thematerial to be replaced.
Check whether the units of measure for the new material need to bechanged.
A material master can only be changed in consultation with theresponsible area.