A lot has changed since SonarQube 6.*, including rule keys in the case of Java. You’ll probably have better results by reading the XML file yourself and doing this manually.
I read the XML and searched the rule keys/key phrase in SonarQube 9, but it’s not efficient. Most of the time, can not find anything in SonarQube 9. Like you mentioned, both keys and key phrase names have changed a lot.
l am hoping there’s a document that can be used to translate/map SonarQube 6 to SonarQube 9 keys during my manual importing rule process.