Re: How many more ALPHA’s before BETA?
I’d just like to say that _ck_ is well qualified to answer questions about just about every aspect of bbPress, including this topic. I can’t fault the information she gave in her replies here.
You should be able to upgrade from anything as far back as perhaps bbPress 0.8 to any later version. Upgrade paths are generally kept smooth, but downgrading is an issue.
I’ve looked into making 0.9 compatible with WP2.7, but the changes it would introduce would probably make 0.9 less stable than 1.0-alpha and take a long time to complete.