If you drop a Clustered Index, all of the NCIs will rebuild because all NCIs contain either the Clustered Index Key or, in the absence of a CI, they will contain a rowID for the HEAP. So far as the performance impact goes, it depends. If a query is using the CI columns of the NCI and those suddenly go away, the index might not even be used for the query which might result in a table scan unless a different index is sufficient and is picked up by the query.