Can user switch from Joomla registration to Facebook registration?

I'm not exactly what you mean by switch. Once a Joomla user is registered on your site, there's a few ways to connect their Joomla account to their Facebook one. Once done, they can log in either via Facebook or Joomla in the future:
1) If the user's Facebook email address is the same as what's on their Joomla account AND you have "Automatically link users by email address", if the user clicks the Login With Facebook button, their accounts will automatically be linked.
2) With Full Joomla User creation enabled, when the user tries to Login With Facebook, they can enter their current Joomla username/password, and that will link their accounts.
3) If logged into Joomla already, and using the JFBCLogin module, you can enable the "Allow Reconnections" setting. Then, if the Login with Facebook button is clicked, their accounts will automatically be linked.

Hope that helps, but if it's not what you were looking for, just let us know!