Name for journal article that was extended from a conference article?
First of all, I do not think there is any etiquette here. The only rule to follow are the instructions of the journal guest editors. If they do not mention anything about the title, then you can change it. 

Now, if you ask about common practices, I can tell you that, in my experience, it is common to change the title, for a number of reasons:


It is a different publication, not a republication of the same paper. You have mentioned that it is an extended version, and I guess that you are suggested to include a percentage of new material, so a new title is in order. 
As you point out, it avoids misunderstanding. For instance, when including both publications in your CV, you do want the evaluators of projects or posts to distinguish them.
Note that, even if you use the same title, you could be suggested a change by the journal referees (I assume that the paper is subject to the journal reviewing procedure).


I often have read the explicit instructions "you can change the title and/or coauthors..." but, in case of doubt, there is no harm in asking the guest editors. 

Personal note: I once used the same title for the journal special issue and I regret it, for the reason 2 mentioned above.