-7024: Alter column not allowed, constraint violation 

Explanation:

An attempt was made to redefine a range of values for a particular column. The SQL statement fails when the table contains rows that violate the CONSTRAINT definition.

User Action:

...

       1.      Choose another CONSTRAINT definition, or do not change the definition at all.

       2.      Use a QUERY statement to find out which rows do not meet the CONSTRAINT definition. You can then change or delete these.