Major challenges would be component, script, dependencies, api difference.
When a API is deprecated, it's easy to make it available in future version so that at least upgrading a product does not break anything.
But what you are referring is backport a lot of component, script, dependencies, api difference to two version back.
It's not impossible as long as you are willing to do the hard work of converting all things in 6.3 to compatibility of 6.1.
but if we retail uses a specific feature or repository of 6.3 which was not there in 6.1, that would one more bigger challenge.