Hello Halcyon,
Welcome to the Community page,
You will be able to restore the 2015 backup in 2016 if you log in as the admin user, however after opening and updating the file in 2016, you would then not be able to use the newly restored 2016 company file in the 2015 version. The backup will be opened in 2015, but if you restored the backup on 2016 and added transactions, those would only be viewable on the 2016 company file.