Just to add - aside of what @Akash_N_Jain said about the Studio itself, the activities, even though they’re separate, changed too and since core packages have studio version limitations (when big changes happened) you will not be able to use the newest versions in 2016.
ForEach has an additional Index property which will make it not open with old packages.
InvokeWorkflowFile has a Timeout property that will act the same etc.
You could go through all xamls and modify them to fit, but it would still not guarantee working (and at this point you might just rewrite it in 2016).
The fact that activities are named the same, doesnt mean they are the same - changes under the hood that could and did make them work differently will bite you hard.