ok, I responded to this without double checking the DNN 3 table structure, the membership system changed from DNN 2 to DNN3 - the major thing is that passwords are now encoded. Because of this I don't know what to suggest and it will take me a couple of hours to research, so I would have a look at the links below for suggestions & request that when you find the answer to let us know so that we can share the information with the rest of the community.
I would not attempt anything that you are unsure of on your live server, it is just not worth the risk. Test it on a localhost install - get a localhost install setup, you will always find it useful for testing certain elements.
You may be able to find help with the bi4ce team that I interviewed at:
http://dnn.bi4ce.com/Forums/tabid/2...fault.aspx
They have a lot of experience with working with the dnn database & may have already solved this problem.
The other resource I would go to is the asp.net forums:
http://forums.asp.net/90/ShowForum.aspx
Hope this gives you a starting point,