Just in case anyone else is looking for this info, I have just gotten it to work. Inspired largely by: http://thepihut.com/blogs/raspberry-pi- ... berry-pi-2
but which is missing a few pieces of info for certain situations.
You will need another computer capable of mounting the microSD card (possibly with an adapter) to carry this process out as described; it may be technically possible to do this "bootstrap style" from a running Pi, but that's another layer of complexity I've not gotten sorted out yet. I personally just performed this on a Mac, although AFAIK it should work on most modern computers. Ideally both this and the Pi have network connections, although it's possible for them to share or to do it in other ways.
You will also need an existing Pi supported by the card image you have already. Presumably, if you have a card image you want to keep but upgrade, you have an existing Pi you were using it with in the first place. But if for example you had a Pi A+ in your birdhouse and a new Pi 2 indoors, you'd need to do some juggling or borrow another old Pi to get everything updated the first time.
* Strongly recommended: Back up your microSD card before starting.
* Boot the microSD card on a Pi that it already supports; if you need to update from pre-1.3.12 NOOBS to 1.3.12 NOOBS or greater for Pi 2 support, this will need to be a "version 1" (non-2) Pi.
* Log in normally
* Make sure you have a valid network connection
* If necessary, set the date & time or wait for it to update correctly from the network
* Update Raspbian (this can take quite a while, tens of minutes on an OK network connection and longer on a slow one):
** sudo apt-get update
** sudo apt-get upgrade
** sudo apt-get dist-upgrade
** sudo apt-get install raspberrypi-ui-mods
(this is probably optional, but brings it much closer to what you'd get if you installed fresh)
* Recommended: reboot the Pi (sudo shutdown -r now) and make sure everything still works
* Cleanly shut down the Pi (sudo shutdown -h now)
* Remove the microSD card, place it in an adapter if applicable, and plug it into your actual PC (Mac, etc.)
* You should see N partitions called "BOOT", and 1 partition called "RECOVERY"; where N is the number of different operating systems you picked to install in your original NOOBS install. E.g. when I first set up this card using the NOOBS installer I set it up with Raspbian, RaspBMC, and a spare data partition (which doesn't count), so N=2; there were two partitions called BOOT along with one called RECOVERY.
Each operating system has a BOOT partition. You should be able to recognize the Raspbian one, because it has recent dates in it because you just updated it. It might be a good idea to create a text file in there "this_is_raspbian.txt" or some such for later reference.
* Download the "NOOBS Lite (Network Install Only)" version from http://www.raspberrypi.org/downloads/
This should be a fairly small ZIP file (20-ish MB).
* Unzip this into a temp directory (making sure the software you use to do so preserves directory structures)
* You are going to "ignore" the helpful instructions (such as the file called RECOVERY_FILES_DO_NOT_EDIT)
and overwrite *some* of the files in the RECOVERY partition as follows:
** Copy everything (files & folders) *EXCEPT* the file called "recovery.cmdline" from the unzipped temp directory, into the RECOVERY partition. These should directly overwrite similar files and folders already present in most cases. If your OS prompts you whether to overwrite, tell it yes.
* Dismount *ALL* the partitions (N x BOOT + RECOVERY) cleanly from the PC, remove the microSD card, and you should be done.
* You should now have a microSD card that works in either a version 1 or version 2 Pi; it should boot NOOBS normally and successfully boot into your updated Raspbian.
* Updating other OS images *probably* can be done in a similar fashion, if there are versions that support both version 1 and version 2 Pi; but I haven't tested the details.