i have modified the old SQL changepasswd Plugin from JohnDoh for using with ldap this is easy.
this is not api based at the moment i planed to code this as api based plugin.
You can try to modifie the SQL changepasswd plugin from johndoh see
RoundCube Patches & Pluginsor use the old styled plugin and change the save_changepasswd.inc and add the save password routine for ldap
my version has some changes special for my case and rc instance this works not with a standard RoundCube.
after vacation i will work on the api based plugin without special modifications thats the plugin will work with a standard roundcube.