Yes, we'll be leaving an 'old' branch for 1.7.
In regards to compatibility, we've done quite a bit of work to make old saves compatible and added a bunch of logic to handle new provinces in old saves and give them correct base tax, appropriate culture and owner, etc, but we're not guaranteeing that old saves will work, so you use an old save in a new game at your own risk.
In regards to compatibility, we've done quite a bit of work to make old saves compatible and added a bunch of logic to handle new provinces in old saves and give them correct base tax, appropriate culture and owner, etc, but we're not guaranteeing that old saves will work, so you use an old save in a new game at your own risk.