i would say it's a replacement, with different design (I'm not a optic engineer, I'm not very sure abt this, just wild guess). Like nikon 18-200mm with 2 versions, and the various 70-200mm from nikon, canon or sigma.
18-270 VC replaced 18-250. Reasons I cannot tell you, because I do not work in Tamron. But I think one of the factors must be the stiff competition from original brands as well as companies like Sigma making the same lenses. So add VC, add 20mm.