Is it possible to send an extended version of a low-tier conference paper to 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.