You would need to get to a point where you are comfortable and knowledgeable about both distributions, versions of asterisk and Freepbx, config files and databases. Not to mention Linux and Webmin.
Additionally I'd try to distill down exactly what elements and functionality need to be preserved from your current enviroment. Each one of those will have associated modules, config files, feature codes or database elements that will need to be migrated.
If you are really serious about this you could either set up a machine where you can run as many virtual instances of PIAF as you would need to experiment with or more conventionally set up two PC's. I see the two PC's being used as follows.. the first would be the "pre-production" machine you meticulously build with layer upon proven layer. The second machine is the test box that you'll probably rebuild and configure many times while transferring the proven elements over to the first machine.
Depending on whether this is a personal or business venture you could also consider some professional consulting assistance from someone in the forum, rent a consultant, farm it out as a quoted project or tap into the Freepbx folks.
It may be tedious but I'm sure you could eventually set this up as you desire. I'm obviously partial toward PIAF but you need to examine your own reasons for the change both personal and technical...
Regards,
Brian