Skype say that the updated version of the Sype app for Windows Phone 8.1 will be released in the "coming months". That means it should be available at roughly the same time as the platform update starts to roll out to existing devices.
Upgrade calls to video and an updated SIP stack
The "upgrade" call functionality is built directly into the standard calling screen of the phone. A single tap will transfer an ongoing cellular call to Skype, essentially offering an easy way to upgrade to a video call. Self evidently, to use the functionality you must have Skype installed and set up on your phone, and the Skype user name of the person you are speaking to linked to the relevant entry in your People Hub (i.e. in your phone book).
In order to make the handover as seamless as possible, Windows Phone will maintain the cellular call in the background until the Skype call has sucessfully connected. The idea here is to make the experience as seamless as possible for the user, automating what would otherwise be a somewhat awkward process.
During Build Microsoft made it clear that the Skype integration, like many of the new features in Windows Phone 8.1, was built on the same APIs as those available to developers. This means it will also be possible for other VoIP based services to integrate with the calling screen in a similar way to Skype. This could include both big name third party services (e.g. Viber) and operator specific services (theoretically both RCS and OOT, such as joyn and TuGo).
From a technical view point, the new functionality is understood to be implemented as part of Windows Phone 8.1 improved SIP stack and related functionality. Microsoft has not offered much in the way of additional details here, but the improved SIP functionality also holds out the promise of improved integrated with corporate telephony systems and, potentially, next generation telecoms services.
Integration with Cortana
Skype on Windows Phone 8.1 will also be integrated with Cortana, Windows Phone's personal digital assistant, by way of the voice control functionality. For example, it will be possible to say "Skype, get Joe Bloggs on video" to kick of a video call to the specified person. This is an example of the kind of third party app integration that Cortana allows for, in this case an evolution of Windows Phone's existing voice command functionality.