Você não especificou
changetype: modify
e
replace: givenName
Deveria ter sido:
sudo ldapmodifyuser 9928892
# About to modify the following entry :
dn: uid=9928892,ou=Users,dc=thisplace,dc=com
objectClass: inetOrgPerson
objectClass: posixAccount
objectClass: shadowAccount
uid: 9928892
sn: FUJI
givenName: GABUTO
cn: GABUTO FUJISHITA
displayName: GABUTO FUJISHITA
uidNumber: 18055
gidNumber: 5000
gecos: GABUTO FUJISHITA
loginShell: /bin/bash
homeDirectory: /home/9928892
userPassword:: e2NyeXB0fS...
shadowLastChange: 17575
# Enter your modifications here, end with CTRL-D.
dn: uid=9928892,ou=Users,dc=thisplace,dc=com
changetype: modify
replace: givenName
givenName:GAKUTO # Ctrl+D