> You can not convert a hdb backend into a mdb backend without changing
> the underlying database. slapcat(8) the hdb database into a file and
> slapadd(8) the file into a mdb backend.

Hello, but it works. I have made the following changes to the config.ldif file.
Then I replayed the LDIF file into an empty ldap directory with slapadd:
olcModuleLoad: {0}back_mdb

dn: olcBackend={0}mdb,cn=config
olcBackend: {0}mdb

dn: olcDatabase={1}mdb,cn=config
objectClass: olcMdbConfig
olcDatabase: {1}mdb
olcDbMaxSize: 1073741824
structuralObjectClass: olcMdbConfig

Thank you in advance for your support