So we need to have a script to update all event logs (scala way) but that will not upgrade the configuration repository. 

Best way to upgrade all to a new fileformat in config-repository is to proceed and full export when starting rudder in 2.8

Another quick win would be to unserialise Rule by putting default value to the Rule 

    (xml \ "category").heatOption.getOrElse("rootRuleCategory")

instead of launching an error.

but that would mean that we handle missing datas, which is never the case for all unserialiser ...