Is it possible to send an extended version of a low-tier conference paper to a high quality journal? In Computer Science, it is common for extended version of conference papers to be submitted to journals. In this case, is it relevant which conference the paper was published in, for acceptance to a high quality journal? For example, is it possible for the extended version of a good paper from a low-quality conference to be accepted by a high-quality journal?
The short answer is: yes.

The journal review process is generally completely independent of the conference review process.  You will likely face additional skepticism from the reviewers, who may well be rightly prejudiced against the original publication venue.  If your paper really is solidly good enough for the journal you send it to, however, it's likely to do just fine in review there.