Should I review the journal extended version of the same paper I reviewed for a conference?
I see no problem...

People re-review the same content all the time.  This applies to resubmissions of rejected conference papers but also journal articles.  In fact, I'm pretty sure I have been asked to review some journal submissions in the past specifically because the editor (who might also have been the program chair for the conference where the earlier version appeared) knew I had already reviewed it.  

In this case, if you felt it was preliminary at the time of the conference, that does not by itself mean the extended version is unsuitable.  You would have to read the extended version and decide.  But just because you didn't expect to see a version suitable for the journal doesn't IMHO mean you're so biased you can't review it objectively.

P.S.  I'm pretty sure I saw a question similar to this in the past few months, but I'm not finding it.