It is always preferable to do a fresh install if that is possible.
Yes. Read this. Those instructions are for upgrading 4.3 to Rolling but the procedure to upgrade Rolling to Cooker is the same. Also read this
It has been about 2 weeks since I last tested this, but it worked for me without incident about 2 weeks ago in 2 different tests.
Cooker-warning: It is OK to use these ISO’s with Cooker repos for a few weeks of testing. Most users will not want Cooker long term. This is due to the nature of things that happen in the normal course of developer work.