Should I review the journal extended version of the same paper I reviewed for a conference? I acted as a referee for conference XXX where I reviewed a paper. It was decent and suitable for the conference, but I recall I thought it was too preliminary and lacking future projection for being selected for the special issue of journal YYY (which I was aware is prepared every year).

Now, I have been invited by journal YYY to review the extended version of the paper from conference XXX (at first, the title sounded familiar, but then the authors themselves declared it was an extended version).

So am I somehow biased against the paper?, or maybe am I the best suited to review the paper since the topic is already familiar?

As a practical answer, should I accept the review?, should I disclose the circumstance to the editor? (who I think would not even care)

Finally, as an hypothetical case, if my review at the conference had been deeply negative, is it still reasonable that I accept the journal review? (in this case being truly aware that I am strongly biased against the work).
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.