This depends on how complex you want the transfer to be--If you want to transfer everything each user has done (e.g. submission history, comment history), you won't be able to just copy over tables because so many table structures have changed between versions. But if you just want the users themselves to be in your new OCS instance, transferring over the users, user_settings, and roles tables should be pretty straightforward, and should work just fine. Since this is uncharted territory, I suggest making backups of all your databases and just doing a little experimentation!
Public Knowledge Project Team