Conference organizer pushes to Journal instead of Conference Proceedings
Publishing peer-reviewed conference proceedings in an associated journal is a trend gaining traction in CS lately. Many universities are pushing quantitative evaluation of researcher output and punishing conference-as-publication fields, so this is to help researchers make the case for their productivity to the layers of administration that aren't familiar with the venues of a given field.

But your case sounds odd, especially since you weren't told beforehand, and the journal doesn't seem to be affiliated with IEEE at all. Definitely contact the organizers and make sure this isn't some scammer pretending to be them to snipe the paper out from under them...

If they really have chosen to change the publication venue, you don't have to accept it: you can always decide to withdraw your paper instead and submit it elsewhere.