Is OS upgrade from Ubuntu Core 15.04 to Ubuntu Core 16.04 supported? Will an 15.04 application snap work on 16.04 ubuntu core .
To upgrade your system from Ubuntu Core 15.04 to Ubuntu Core 16 you will need to contact the manufacturer of the device you are targeting. They will have a migration procedure available.
A 15.04 application snap will not work on Ubuntu Core 16. There are migration guides available to help port it over.