Yes.
A database index by definition indexes the data in a record.
To not change the index when its key is modified in a record occurrence would corrupt the database.
For optional indexes (i.e. anything other than
MANDATORY AUTOMATIC), MODIFYing the value of a field which is in the index key does nothing to change that record's membership of the index.
If it is a member, the value will be changed in the index and the record remains a member of the index.
If it is not a member, the value is changed in the data record but there is no change to the index.
The only way to change a record's membership status within an optional index is with the
CONNECT and
DISCONNECT verbs.