If you turn off your forums, then users will not be able to register since it requires your boards to register. Changing integration types would require you to write an "import" script to move your user database over to the new forum.
As for migrating, as long as you are running PP5 you'll be able to do a conversion to another board - as long as the USERIDs stay the same between conversions, there is nothing you'll need to do for the PP side (that I can think of, depends alot on which forum you switch to).
Please do not PM me for support or sales questions. Thank you for your understanding.