Web Designer MX Premium will install as a separate application.
There is no upgrade path from Designer Pro 7 to Web Designer MX Premium because Web Designer would be a retrograde step from the flagship Pro version.
The next version of Designer Pro (likely will be similarly named, Xara Designer MX Pro) will, going by past years, follow 4 or so months after the Web Designer release. And yes, all the new features of Web Designer MX Premium will be included in Designer Pro (MX)