Speaking to Google at the Android team's office hours event, we learned that Android N's new seamless update feature, borrowed from Chrome OS, will not be introduced on any current Nexus devices. While the benefits of seamless updates' usage of dual system partitions to allow updates to the OS proceed in the background are substantial, they're also a bit of a technical bear to put on existing smartphones. Implementing seamless updates on the Nexus 5X or 6P, for example, would require repartitioning the entire phone, and wouldn't be possible for an end user to complete without hooking the phone up to a computer, allegedly - it would just be a pain, in short. While references to seamless updates have been found in the latest N developer preview, it was confirmed to us that these have no bearing on existing devices getting seamless updates.
As such, it's exceedingly unlikely we'll see seamless updates implemented on any existing Android devices by any manufacturer, and instead that we'll only see it on new hardware running N or above out of the box going forward. For example, on new Nexus devices later this year.
Seamless updates borrows an update method from Chrome OS in which two system partitions live on the device at any one time. While one partition is in use by the phone at any one time, the other is free to be updated by the system, so an OS update can be downloaded and installed completely in the background, at which point the user reboots the device and the update will feel little slower than, say, a hard reboot.
While it's a bit disappointing that current devices won't get it, the practicalities of the situation (ie, the potential of someone inadvertently bricking a $500+ phone) are what they are, and it's easy to see why Google wouldn't go through the trouble - there's really no good business reason for them to do so. But hey, it's something to look forward to, just not on current devices