By means of import I changed and existing SSL entry to become a valid certificate. The interface allows me to do this, so you suppose that LC will take care of the rest. However I did not see the certificate of the associated domain updated until I entered the domain section and cycled through removing and adding the SSL configuration. Only then the apache configuration was updated and the changed certificates became active on the domain.
I don't know what happens behind the scenes, but apparently the old cert files remain while new ones are written. I have restarted Apache manually and the old certs remained valid, hence my conclusion that the config files were not updated. May be it would be better when the files keep the same name, because then an Apache restart would be enough (or even not necessary?).