The functionality is currently broken (see bug 8899), and it will become practically impossible to maintain once we fix bug 7880 and merge the image customisation feature.
Created attachment 2928 [details] Design - Remove the ability to change the project release
Patch submitted for review https://lists.yoctoproject.org/pipermail/toaster/2016-February/003834.html
Patch merged to master http://git.yoctoproject.org/cgit/cgit.cgi/poky/commit/?id=07eb4f274ca4f002a9280b2cf6e7b81bc17c4ed6
Verified on master.