1. Install / upgrade to openldap-2.3.21-r1
2. Try to reemerge openldap (in my case part of emerge -e system)
Being the same version there is no special upgrade procedure required.
emerge dies with:
* A (possible old) installation of OpenLDAP was detected,
* installation will not proceed for now.
* As major version upgrades can corrupt your database,
* you need to dump your database and re-create it afterwards.
This is because /var/lib/openldap-data/.version-tag stores the version including the revision, so the ebuild will only find "-r1" as the installed version.
Manualy changing .version-tag to say OLDPF='openldap-2.3.21' fixed it.
Fixed in 2.3.24, thanks for reporting :)