There are converters from one format to another for some of the systems, but there are no guarantees that moving from one specific system to any other is possible/easy.

You might also want to explore the situation for integration of your forum software of choice with other software such as portals, content management systems, etc. Anything you think you may require in the future. Of course you can always contract for the development of a custom solution if nothing exists now, but it's always nice to know your options.

Bruno