There are many converters available to help you migrate your current forum to SMF without losing your old members, subjects, messages and other things. Please visit Converters and Tools for a list of currently available converters.
Try also browsing the Converters and Importers board if you have any question or trouble in converting your forum, or to request the coding of a converter which does not exist yet.
For documentation on converting, please see the Converting to SMF document.