Thanks to those that have tried the new beta and given us feedback. We've already fixed several bugs; things are shaping up nicely!
About the new "Anvil" level format, we will be supporting this, but it is a rather hefty change. Our tentative plan is to make a new branch off of the beta "rewrite" branch for the anvil level format. Once the anvil format makes its way out of the Minecraft "pre-release" stage and into a proper release, we will merge the branch of our code and the new format will be the only one supported from then on.
We currently don't have any plans to support the Anvil format in the old "master" branch. We are not against it, but I don't foresee anyone stepping up to do the work. This means there is a deadline for those of you to convert your config files and setups to the new config format if you want to stay up to date. We believe the new config format is not complicated enough to be a big barrier for converting. If you need help please see us in IRC!
We also currently don't have any plans to support both world formats in one code-base. The changes it requires are quite extensive and I don't see that as being a very feasible option. Thus, if you want to continue running an older version of Minecraft with the older level format in the future, you will be blocked from updating Overviewer past a certain point. We will probably end up leaving a Git branch or tag at the last commit that supports the old format for those that don't want to upgrade. However, as time goes on we think most everyone will end up updating (just like we did for the old chunk format when it upgraded to region files)
Again, this is our tentative plan. Things may still change depending on demand or who's willing to do what work or how difficult a task turns out to be.
As always, happy Overviewing!
-brownan