I don't think it is just the interface, although that's the most obvious. Sometimes, it is easier to tear a building down and build a new one with a proper architecture, rather than adding floors on the old one and just hope that everything is not going to collapse. If PTC is at least a little bit sensible (how naive of me to still think that ...), they must have taken it appart in order to have better foundations for future improvements.
Except that there is no way to know if the foundations are good enough to support a taller building, since the new one is smaller than the old one.
From what you guys say, it looks very much like we are currently under Prime 0.3 ...