"(2015-03-01)" in your uname --all output is the date of the kernel compilation, probably.
Debian will not automatically remove your existing kernels on upgrade.
Every release has its own default binary kernels, which all correspond to a single kernel version, but of course are built for different architectures. It is generally a good ideas to use a default kernel for a given release.
When upgrading to a new release, you can manually install the new default kernel, but a convenient way to obtain this kernel is by installing the linux kernel meta-package. This package's name is of the form linux-image-<arch> where <arch> is whatever your architecture is. So, in the case of amd64, it is linux-image-amd64.
That meta-package is designed to always depend on the current default kernel, for whatever release/version the meta-package belongs to. Therefore it will pull in the current default kernel as a dependency.
However, your current kernel will remain installed, unless you remove it yourself, of course. You will need to reboot to switch to the newer kernel.
Also, you should also make sure that if you have unstable or experimental sources installed, you have a suitable entries in your /etc/apt/preferences file, otherwise your packages will be upgraded to the unstable/experimental versions. But that is not directly relevant to your question.
Ubuntu releases aren’t based on Debian releases. During the development of an Ubuntu release, packages are imported from Debian unstable, until the Debian import freeze (in the past, LTS releases imported from testing, and this is what the linked wiki page still suggests; however looking at my packages shows that 18.04 is importing packages from unstable). This means that a given Ubuntu release will have non-Ubuntu-maintained packages in whatever version was in Debian at the time of the import freeze (barring explicit sync requests); but that doesn’t match what the next release of Debian will contain.
So trying to tie a release of Ubuntu to a release of Debian would just end up being misleading.
You can look at the contents of /etc/debian_version to see the Debian codename of the version (under construction) from which packages were pulled; you can also match Debian import freeze dates from the release schedules (for example, Artful’s, Bionic’s, Cosmic’s, or Disco’s). You’ll see from this that the same Debian release feeds multiple Ubuntu releases (e.g. Stretch, which ended up being Debian 9, fed Xenial, Yakkety, Zesty and Artful; Buster, which will end up being Debian 10, fed Bionic and Cosmic, and is feeding Disco), with quite different package versions each time.
Best Answer
While it is still not a given, officially most probably the last quarter of 2016, with the release of Debian 9. In the meanwhile, you can start using testing, compile it yourself or using a version compiled by someone else. I am using armbian in a Raspberry Pi like-device (Lamobo R1), which is Jessie, and using a v4.x put together by the armbian guys. On my Intel servers at work I plan to go soon to v4 too with Debian 8.