Normally you can rename any Blueprint, variable, etc in UE4 and it does a fantastic job of refactoring all your references for you. It makes development life amazeballs. Unfortunately, this nirvana does not extend to renaming BlackBoard keys.
Thank you for the report. I was able to reproduce you issue in the 4.7 Preview. I created JIRA UE-8415 in our tracking software and our developers will be investigating it further. We will post back here with an update as soon as we have something.