They will mostly update, some people have had a few issues. CC will be a fresh install and you can keep (as I have done) CS6 for any older projects. If you work with 5.1 audio you can upgrade the Surcode encoder to CC but you will lose its function in CS6.
BEST PRACTICE: Complete current projects in THEIR version, and start new ones in the new version. Staffer Kevin Monahan (former life a major professional user/teacher of FCP before becoming an Adobe staffer) who frequents this forum preaches that as THE proper work-pattern in ANY NLE or video app for professional use. You gotta be able to get that work out, and though projects MAY transfer forward just fine, if they glitch up and you didn't keep the old project pristine ... you're screwed. Many people have up-graded projects without a hitch, but if you go through this forum you'll find a number that ... um ... it wasn't a happy moment. Or week. Or month.
His basic recommendation ...
- Any projects that will have a "final" state are worked to that state in the "old" version.
- The newer versions can be downloaded and installed separately from the older ones ... use them for any new projects and if something or some effect doesn't like the new version, you can still run the old one.
- Projects that are always on-going (like tv series) should be extensively tested with a COPY of you previous-version project and then ... when any issues that MAY pop up are resolved, move forward with the new version on its new project. NEVER allow the "main" copy of the old project to be modified past it's version.
- Keep past versions on your computer, perhaps renaming the icons so you know which programs "match with each other, across the PrPr, Ae, Sg, Audition, and AME set.