If you read the Seattle Times article I linked, it says it's expansion - they're going to move some of their existing 747F to domestic service and operate the new -8F internationally. Six of their existing 747-400F are relatively new - they picked them up when Boeing offered the last 747-400 production slots at fire sale prices after the -8 was launched.
However I see they also have a couple 747BCF - it wouldn't exactly surprise me if those got parked was the 747-8F came on line. I just retired from Boeing so I can no longer check the hour/cycle numbers but those BCF's tended to be rather high time and their operating economics are not as good as the pure freighters.